Canonical provides transitional packages for packages that they've decided to provide as snaps. They're not forcing anyone to use snaps, they're saying "if you want the default we provide you, we're providing you with a snap." KDE Neon (my current distro, which is downstream of Ubuntu) has decided that they want to use the deb packages from packages.mozilla.org, so they provide an override. If you want to use the deb from packages.mozilla.org, you could grab KDE Neon's repository deb and install that, or just set up the mozilla repository and use the same pin file they already have.
This is like saying "Debian FORCES you to use libav" Debian moved from ffmpeg to libav for a while. No, they provided libav and made transitional packages for this drop-in replacement. Some people didn't like that and made their own ffmpeg repos, and the process for using their separate ffmpeg rather than Debian's transitional packages was the same as the process for using Firefox from a different repository. (I was one of the people used some third-party ffmpeg repositories, and I was glad when they switched back to ffmpeg and provided libav to ffmpeg transitional packages.)
Does the fact that the Ubuntu repositories don't contain Keysmith mean "Ubuntu PROHIBITS you from using Keysmith?"