Why should vod unfairly get the monopoly on Peter Griffin boob-licking avatars?
Seriously though I don't think we should limit people to unique avatars. Users may use the same ones to indicate they're from a specific company (or crapcoin or whatever), and like mprep said free speech and all that shit.
Another additional argument: additional server work and/or space. Every time someone uploads an avatar, it has to be compared to all the other avatars in the forum pixel by pixel (or compared to their files hash or whatever else).
If it's compared by hash then the hash has to be stored as well (although the size probably wouldn't be too big in this case).