Penn got 6 months because he essentially admitted to not doing paperwork. USADA banned IVs because they can be used to enable and mask blood doping. If he wanted a one time IV, they could have tested him before and after, or just given him the TUE and been done with it after getting documentation from the doctor. Penn was dumb about it, and I have my doubts about whether his treatment would have been effective, but there was no indication that he was trying to cheat or gaining some unfair advantage by getting an IV. There's a pretty long history of USADA issuing short suspensions and TUEs going forward for minor infractions of the WADA code.
Crocop got 2 years because he admitted taking HGH, which he'd have a hell of a time getting ever getting a TUE for.