Right.
"it is clearly malicious behavior and may fall on the wrong side of the law."
It's not against the law. It might be _shady_ but it's not illegal. When I'm teaching cyber intro classes, I let folks know portscanning is NOT illegal but shady. It's like going to a business after hours. It's not illegal to rattle doors and windows to see if they're locked. The police might have a different take on it, but it's not illegal.
I don't think you need to overdo it in terms of making the warning red, etc. Just a popup will really discourage people from trying to use this for fingerprinting.
BTW the site says:
"Port scanning is malicious." I don't agree. There are many many things that can look like a port scan but are not malicious, most notably NAT traversal attempts by WebRTC, games, chat apps, and so on.