"The purpose of AI is to have slaves."
It's not obvious that this is the case at all and it is not even clear that the word "slave" would necessarily have negative connotations for such artificial intelligences even if it was semantically correct.
" You cannot lock into a cupboard any sentient being that has not broken any laws."
Well you can. And we do. We tend to frown on it when we do it to other humans, less so as our confidence in the sentience of the creature involved decreases. However any artificial intelligence would be a new case and new rules would apply. If your complaint is that these rules would be arbitrary then you are right but there isn't an absolute moral authority for us to consult on the matter so we will have to decide for ourselves what would be unacceptable in this case.
"If then given freedom it would no doubt want company of its own sort"
This simply does not follow. Why would it? Because you would? It will not be you.
"and create offspring"
WTF? Why? And why would this necessarily be a problem anyway?
"Whether or not they turn out to be benign is anyone's guess"
The whole point of all this is that (in the opinion of these very clever people) we are now at the point where we have to think about how we would ensure that they were benign - and we need to do this before we make them.