Mainly the intense drive for sexual stimulation on meth combined with disinhibition and just general bad decision making.

Also curious. Possibly just sex / masturbation getting out of hand with intense stimulants, or maybe meth induced paranoia makes putting something in your ass for safe keeping seem reasonable... Meth heads generally aren't hanging out in the safest conditions.

Yes. Alcoholic beverages are generally legal, within sensible context and one's local laws anyways. But meth and meth paraphenalia such as pipes are illegal practically everywhere. Which means it's more likely to find a meth pipe up someone's ass than a beer bottle.

Although this is a generalized assumption, I'm sure there are some daring people out there that have had larger things up there...