"putting in the collectors cans"

Yes, this is a recent thing.
If I've already bought tickets for concerts, workshops etc, I usually decline to do this out of principle. I don't mind buying the occasional raffle tickets, however.

However, the organisers are probably quite right to try and gather some money from those who only visit the pub sessions.

Of course, musicians and singers who attend sessions are contributing as well and there is an argument for supporting them too. The odd free beer or two is always welcome.