It'd be funnier if everyone else in the crew got a bounty but him - think about it for a minute (possible forshadow) -
He's wearing Soge-king costume, marines don't really know it's him. If this gets all said and done, boutnies go up, new bounties formed, etc. - they'll use the same picture of luffy again - with usopps head in the background - so making them all their, but not a set one for Usopp. Since Usopp wants to be a brave sea warrior - he then feels the need to earn one, and continues with the crew to "prove himself".
Just a thought on the whole bounty thing.