renoah you're not listening, i'm sure you won't listen to this either but i'll try anyways
your employers agreement with other local business will just be an informal arrangement e.g. not legally binding so even if they changed their minds and decided to charge you entry fee they could at any point (when i have organised one of these i believe the line i used was management reserves the right to alter these terms at any time without prior notice - or something along those lines).
regarding your main question regarding the charity fund raising - you are not being forced to donate, the establishment have simply decided to make it a condition of entry - i assume that everyone else in the bar had also put money into the bucket so they made you to do the same so that they could make the maximum amount for charity e.g. they were willing to forgo their profit by allowing you free entry (normal entry fee) but were unwilling to forgo charity funds by allowing people in that didn't pay to charity at the risk that they might run out of room inside and have to turn away fee / charity paying customers.
as said you were not forced it was your choice and you chose to give to charity - perfectly legal and a nice / commonly done move when a bar or club want to raise charity funds
i will expect whithering scorn for trying to answer your question as a reply
love iggy x