I think the reason a tip is mandatory for large groups is because:

1) The bill is paid by just one person, and a 15% tip on a large amount is a large amount of money. Some people will say "wow, that's a lot, let me add what I think is a reasonable tip", which works out to be only 5%.

or

2) The bill is split up amongst everyone, and it takes too long for each individual to figure out how much their share of the tip is.

Since 2) is basically the same as a party of one or two, I think 1) is the primary reason for having a mandatory tip for large groups. I don't see a problem with it.