I wouldn't have a problem with a sign saying it's $3.00 to sign up for the game, but a tip jar isn't the same thing. With an upfront charge, people know they are expected to pay or not participate. A tip jar seems passive-aggressive in a way. You're not forcing people to pay, but you're basically labeling them as cheapskates or unappreciative if they don't. An upfront fee lets people make their choice at the outset, rather than putting in $3 and then feeling guilty after the guy next to them puts in a $10 bill.