Much of this has already been addressed. In terms of law enforcement, what stops them is the law and what they are legally able to do, as well as you using the Emergency SOS (five-tap of the side button) or powering off the phone, which requires the passcode - FaceID won't suffice. In terms of continuously scanning for a matching face, it has already been identified by Apple that it will scan twice and that is it - after that the passcode is required. It seems hard to believe that anyone would believe that Apple (creators of the TouchID) wouldn't think of these things.
But yes, if you let someone take your phone, hold it up to your face, and you keep your eyes open, they could unlock it. Of course, if they grabbed your finger and put it on the phone, they could unlock it too....
I think you are missing the point
You would likely know if someone was trying to press your finger on your phone, not so much if someone was holding a phone nearby, but close enough to register.
Exactly, after two tries to recognise a face, the password is needed, and hence my point as an owner of said device in a crowded area am I going to be constantly having to insert my password every time I try and use my phone?
As for what Apple think about and considering every scenario I guess they thought SIRI, and maps and the way we hold our phone were never going to have issues?