A couple of thoughts:
Some tire dealers do not like to deal with warranty issues - and especially those where they didn't sell the tire. Since tire shops aren't exclusive dealers, the tire manufacturer is hard pressed to force any particular tire dealer to deal with warranty - and Michelin is no exception.
Discount Tire has the reputation of being easy to work with - even if they didn't sell the tire. Their idea is that a pleasant experience will pay off in the long run (and your experience seems to verify this.)
In theory, Discount Tire didn't need to call Michelin. They could have changed out the tires and pretended they had originally sold them (assuming they carried the particular line of tires - and maybe that was what was complicating the problem.)
Nevertheless, glad to see you were able to get this taken care of.