I can see how a lot of people feel like it's sort of lazy, or cheating. In some cases, these mashups could definitely be done live with just 2 tracks. Obviously, with a premixed mashup, there's no risk of the messing up the timing live, and it might sound better if it was produced. If someone was using it to work in an additional track, I think a lot of people would look at it as an efficient way to mix 3 or more tracks (one being a mashup, and the others being from separate tracks).
On the other hand, some mashups are done really well and in more complex fashion (isolated instruments, for example), without making it sound like a fake live mix. Basically, no possible way to mix it that way live.