IPv6 site local addresses are like private addresses in IPv4: You are not supposed to see these addresses on the Internet and therefore anybody can use them for numbering of a private network. They are not globally unique.

Since the router won't know automagically if it would be forwarding a packet outside an administrative domain, you would have to set up filters yourself at the egress points of you network, just like you do with IPv4 private addresses.

Hope this helps.