The reason we are not permitted to swear is because although there are some elder children on MSP, like you and I for example, there are some younger too. The game is based for kids 9-15 or something like that, so they can't allow 9 year old's to swear. Nor can they allow 15 year old's to swear, as one of the youngsters might see it and abuse the ability to use the words and might make a habit out of saying things that shouldn't be repeated to anyone. Parents, friends, family, ANYONE. 

Hope this answered your question well. As much as I would also be permitted to swear, it'll never happen :/