As many people have already pointed out, I am not aware of any direct mechanism by which it causes acne, but many of its (side) effects are predisposing factors for acne. It's the same as damage to teeth; unless the caustic amphetamines actually come into direct contact with tooth enamel, they do not directly have any effe all cct on your teeth, but apart from the obvious one of bruxism from excessive use, extended periods of dehydration (and a very dry mouth as anyone will testify), poor dental hygiene and a poor or non-consistent diet all contribute to dental deterioration.
I've never had the problem myself but can easily picture the mechanisms by which it might occur and when the speed junkies I know have gone clean, there was a significant improvement in the skin condition of the vast majority of them.
When used in moderation, amphetamines are fairly benign. Their side effect profile just happens to include dehydration and malnutrition, unless you're careful, and their addictive potential often leads to binging, which makes it all the worse. It's more what you do (and don't do) while tweaking than the drug itself, unless your doses are excessive.