No, this is not the expected behavior. When people copy a selection, they intend to paste it somewhere else. They expect to paste exactly what they copied, no more and no less. Whether they trust a website or not should not even be a factor.

People should complain about it. People should complain about clipboard hijacking, hyperlink hijacking and all the other unacceptable breaches of trust that web masters perpetrate. Web developers were given powerful tools and instead of using them responsibly they chose to abuse them. So they shouldn't have access to them anymore.

These issues need to be fixed client side just like ads were. We need to remind these people who owns the machine.