I mean, unless she is some horrible anti-reading sadist, she probably has a reason for saying this which is based on the kids' welfare/smooth running of the class?
I am very opposed to censorship of kids' reading generally, and (within reason) always allowed my daughter to read pretty much what she fancied, even stuff that some might consider too old for her, or too trashy, or whatever - reading is one way to meet and explore the issues of the world before actually having to experience it in gritty reality ....
I don't know the books in question, and if they are aimed at kids I assume they aren't awful. It's not like he was passing round snuff movies and porn.
BUT, she probably LOVES the fact that kids read - all teachers do, even if they don't inherently, because it's one thing they are now judged on (rightly, perhaps).
So one can only assume that the books were causing problems - maybe some highly over protective parents were offended and blaming her? Maybe the kids were reenacting scenes from them, or fighting over them?
I mean, the only way to know for sure, is to politely ask her. But I wouldn't make a big deal of it - it's highly unlikely she intends to be mean.
And presumably if your son really wants to lend the books and his mates want them, you or your partner or whoever picks him up from school could pass them over to the other parents/carers at the school gates?
I was a bit concerned that your son was "crying" about it - sounds like an extreme reaction for 8 on the face of it - are you sure there isn't any coercion or bullying going on, with other kids demanding he shares their books with them? This could even be what the teacher is worried about. Sharing books should be a fun thing for him, not something that reduces him to tears when it can't happen......