I've had loads of occasions where I've thought the same thing - but then I actually checked things out. I'm always getting told off by the pope and told to chill for X turns, then getting really wound up because they're blockading my port and if i attack them I get excommunicated.... but - if I check then there standing has gone down loads, or they've been excommunicated themselves. So yes, the AI can do what they want, but they suffer the same consequences as you.