Not sure if this is a global thing or specific to the UK. A few years back I was working for the Clown and I got invited on a litter-picking day. This involved going out as a team with a manager to supervise to demonstrate how much McD loves its local community. It was unpaid, about 5-6 hours work for 10 staff.
The store manager just watched, didn't help out and the bulk of the litter we were grabbing was packaging from our own restaurant. The only reason I went for it was because I was new and wanted a chance to get to know my coworkers in a more casual environment. If the company really cared they could pay us.