a) Because it won't work:
https://github.com/LudwigOrtmann/RIOT/blob/76b017aefddf5c12e442f55da2c898a8949d1ab7/cpu/native/net/tap.c#L158

b) because there is nothing to be gained.
This check does not guarantee that the payload is intact or anything. This check only makes sure no invalid memory is read upon payload copy. (Think with a malicious packet in mind...)