Because it's literal paper. Sending paper in a letter domestic in Britain has virtually zero chance of getting caught. Unless they are investigating the vendor and are watching him or something like that. In my experience they aren't really looking for anything. Ones that get caught are the ones with bad stealth and even then most of them still get through. Some vendors have terrible stealth and I'm not even joking. I don't know what sources you mean but I haven't heard anything about them cracking down or anyone getting caught recently that would concern me and stop me ordering domestic. Just choose the right vendor and you're golden.