Legally, because everything I've read or listened to from all sorts of "expert" talking heads seems to imply that he's not so much defrauding the IRS as he is using every loophole available to lessen his tax burden. Remains to be seen, but I think it'll be spun.

Politically, because his supporters are stupid enough to cheer him on for being a "smart businessman" (while reveling in the cognitive dissonance around the fact that these loopholes aren't equally available to them)

The debates will be lit.