I think others have already explained this pretty well, but I'll throw my hat in the ring.
My understanding of Methamphetamine is that the drug itself does not cause tooth decay. The tooth decay seems to arise from what you would call secondary side-effects of the drug. Methamphetamine and other stimulants are notorious for causing dry mouth. Saliva is your first and best natural defense against insults to your teeth. The lack of saliva is a major contributor to the phenomenon known as "Meth Mouth".
I think to a lesser extent, grinding of the teeth adds to the problems. Also, as Methamphetamine is a powerful anorexant, people typically go without eating, sometimes for days or even weeks at a time. When the time comes to crash, the appetite will return and your body is going to demand the simplest and quickest fuel it has, sugar and carbohydrates. The desire to eat sweet foods as a result of malnutrition tends to cause Meth users to go for sugary drinks and snacks.
If you aren't brushing your teeth and staying hydrated, then eating sugary foods when you crash, the picture of why Meth Mouth happens really comes into clear focus. I'm not trying to be pompous here, but I feel that this pretty much explains it.