"...if someone doesn't claim that ticket, they're effectively preventing someone else from enjoying the Bop"
You could equally say the same of someone who buys a band, goes in the Bop and then leaves after a couple of minutes. Or, as was certainly common in my day, people who pay for the Bop merely as a means of accessing quicker bar service in the Beer Bar.
"...the reason you can't buy bands for other people is that we want to make sure that we don't let more people into the Bop than we are allowed to by law"
If a record was kept of how many "tickets" had been sold then it would be quite simple to subtract that number from the total number of people allowed in the Bop. If the people had not claimed their band by a certain time then the bands could be resold. People could be required to pre-pay for their ticket. This would not only help to guarantee people turning up to claim their bands but it would also mean that the Union wasn't losing out financially.