The difference is thin, but it is there. "Autant" can only rightly be used when you compare two quantity together. Sure, in spoken language, people will sometimes use both "autant" and "tant", but it's not because people say it that it is right.

http://french.about.com/library/weekly/bl-tantvsautant.htm

I do not disagree on principle but in the case of this sentence in particular, you can say one or the other. Now for the subtlety: this is simply because there is no comparison here. Only an evaluation of a quantity.

you are right. Pourquio boit-il tant de bière? is the right answer

Sorry but both are valid. the difference between "autant" and "tant", here, is as thin as "so much" vs "as much".