I don't think it's immoral to buy those beers, but the monks said you shouldn't so you are going to hell.
They have created a situation where demand outstrips supply so, naturally, the price the market will bear rises. The religious nature of brewery means that they can't be seen to charge more than the going rate, so the just make life easier for themselves instead. They don't deal with distributors, or shops. They don't have to give anyone credit and they don't have to pay any middle men. They just make their customers drive to the monastery at a time that suits the monks and queue up for the right to buy the beer.
Asking people not to sell on the valuable beer they had to jump through hoops to get, is futile and only someone who doesn't live in the real world would think they could do that.