This is a case of は being used as a comparison marker – it can combine with or supplant other particles in a sentence as needed. In fact, more often than not, it’s replacing が, you just don’t notice it. This can give you double particles like には and では. For example, 事務所に行く時、電車では車よりいいですよ – when going to work, it’s better to go by train than by car. Here, the は is performing exactly the same function.

が and を are always replaced by は (so you’ll never get, say, をは), and that’s what’s happening in your example. Don’t forget: は marks the topic of the sentence, not the subject of the verb. In this case, ビール is the object of the verb, and the object marker を is being implied.

Yeah, couldn’t have been explained better.