Passengers pre load because they want to get drunk and have a good time. That's the only reason. It's not the reasonable onboard prices that the catering companies charge which are in line with city centre pubs for all things except the tins of beer. It's more than likely people will buy whatever the prices. Watch a trolley on a holiday flight and you'll notice how slowly it moves - there's absolutely no problem selling the stuff. Pricing is not an issue. Check out onboard brochures and you'll see prices creep up with every new issue. The fact that they know the crew can control them is a strong reason.
Take a look at the full service airlines with 'free' bars you'll see that they do have their fair share of problems.
A lifetime ban is appropriate. If you've shown that you can't behave yourself once onboard an aircraft then there's a very good risk that you could do it again.