He has two bikes in his trailer, a Buell and a Suzuki. He's swapped between the two all year long, sometimes in between races, sometimes during the race itself, based on which combination he feels is better suited to the track, weather, and other factors.
Some people say this is unfair, but he has never broken a rule during any of this. You're allowed to change vehicles once per weekend. So what he's doing isn't illegal, but you could say he is abusing the rule. The one change per weekend rule is there so that in the event of a crash, or other major damage that can't be repaired at the track, the competitor can keep racing. But the rule doesn't say you have to crash your primary car to go to a backup, so again, Matt isn't breaking any rules here. Some also say that he (or anyone else) shouldn't be allowed to swap brands during the weekend. As far as I'm concerned, if NHRA didn't want people doing that, they would have written a rule against it.