"Drinking water" plastic - can I store beer in this?
I doubt that this is a chemistry problem. Food safe plastics which can hold wine and coffee are so abundant and cheep, there is no reason that they went to the trouble of finding one which is not suitable for these, but suitable for water, and made the bladder out of it. 

All beverages beside water are a good growing medium for one microorganism or another. And the bladder is hard to get clean, the sucking straw even more so. So, it looks to me that they are warning you to not put in anything beside water because you'd risk starting your own bacterial or fungal colony. Beer is as bad as other drinks - don't store it there.