It depends on the tire manufacturer's requirements.
This is a good opportunity to point out the difference between a dealer and a manufacturer. Many people think that the dealer (whether he sells tires, or cars, or refrigerators) is the same as the manufacturer - or at least there is a close enough link that the dealer is acting as though he were the manufacturer (or as a surrogate of the manufacturer). This couldn't be further from the truth.
In most cases, the dealer (and again I emphasize, it doesn't matter what they sell) is an independent businessman and is going to behave as such. While he may get paid to do warranty work, he is going to make a business decision concerning the warranty and how it is handled.
In some cases, dealers HATE warranty work: They aren't always paid well for it, there is always a risk that the work won't be approved (so they won't be paid at all!), they have to deal with an irate customer for a problem they didn't cause. Lots a reasons not to like warranty work.
In the case of tires, there are tire dealers who will not do warranty work on tires they did not sell. Many tire dealers will not touch an OE tire - and to some extent that is understandable.
WalMart, for example, has a policy of not doing warranrty work on anything they do not sell (which is one reason why many tire chains have there own unique tires that are virtual clones of what other dealers sell).
Now before I get a bunch of replies saying that they've had warranty work done at WalMart on tires they didn't sell, let me point out that what happens at the store level is not always according to company policy. Usually there isn't a problem, but there have been some circumstances where the store did an adjustment and did not recieve credit for the tire warranty work - and since the company policy is not to handle these - WalMart corporate could not assist the store in getting it taken care of.
On the other hand, many dealers welcome the opportunity to do warranty work - especially on stuff they didn't sell. They figure that if they do a good job, the customer will remember and come back.
I can see both sides of this. In the early days of Tire Rack, there was a lot of resentment at the store level, and lots of tire delaers wouldn't touch a Tire Rack sale - mounting, warranty, whatever! Tire Rack really improved their system by adding a list of dealers would would mount tires and being quite liberal in their adjustment policy.
One other thing. A dealer may do some things for a long term customer that they won't do for a guy they've never seen before. Usually the dealer is acting outside the warranty and never turns in a claim. However, some folks leave the shop thinking they got "warranty".
And in particular, I want to point out "Take Offs" (see the post above)
Aside from the fact that this is not what the Original Poster was talking about, these are always the dealer making a business decision. Usually figured in there is exchange that resulted in these "take offs".
OK, back to the issue of "try Out" tires:
This is a "warranty" program. Technically, it falls outside the "technical" part of the warranty, but rests in the "Sales Incentive" part of the warranty. In order to be sure the selling dealer is actually performing the adjustment - and doing it correctly - most tire manufacturers require the tire to be returned. The dealer recieves a credit based the return (condition, amount of wear, etc.)
In the case of a "Try out period", it becomes a bit complex. On the one side is that the tires may be perfectly good - just a tiny amount of wear. The tires can not be sold as new, but there is some monetary recovery possible.
On the other side - there has to be a reason why the customer returned them. Sometimes it is known, most of the time it isn't. Selling them again just perpetuates the problem - if there is one.
I haven't surveyed the other tire manufacturers, but it would seem to me that they would want the tires returned as proof the selling dealer isn't making this up - basically keeping the dealer honest - as well as to take the tires out of circulation.
Based on what I have seen, very few tires are returned under this "Try Out" program. So either this is working as intended, or the dealers are just taking the hit (I really doubt that!)
The company I work for scraps all tires returned to us, regardless of their condition. This would include tires returned under this program. The reason is that we have a public statement (as does the industry) against selling used tires. It would not make sense for us to testify in court that used tires aren't any good, and then sell them ourselves!
It can be argued that a statement advising against selling used tires is not a good or fair thing to do, but our experience is that there are a lot of folks selling used tires that should never be put back on the road. There are no regulations in this area, and if there is a problem with the tire, the dealer is rarely a party to the lawsuit. In other words, the guy that caused the problem never gets punished. This is a particularly sore point with tire manufacturers!
Bottom line: I'm going to guess that all tire manufacturers require tires to be returned in order to get warranty credit.(with a few exceptions) and this would be one of those situations.