Here's the thing. For some reason, Wii U verifies non-executable files only on installation, just like Wii did with most on-console titles. With DS VC, a non-executable file becomes effectively executable, but there are loads of other types of exploits possible due to above (like game crashing while loading a huge level file). This doesn't sound like something they can afford to start checking now, as #1: Wii U is dead barring Zelda DLC and some VC, perhaps, and its user base is small, #2: Wii U is already slow and verifying files on reading would slow it down further, #3: it could require additional verification helper files for each individual game which would have to be downloaded with the update, somehow.
Updating WebKit is way simpler than that.