It would also go against the cypherpunk ethos, which this forum embraces.

Bitcoin doesn't have rules against one person having multiple addresses either.

That's true. I don't see any criminal behind having more than one account. The question was out of curious, do we have this rule or not 

I don't think so either. And it's difficult to combat this, an IP approach would be one way, but just use a VPN and it's no longer possible to control.