Not illegal, but much harder to find than the sliced, cured belly meat (often from the side of the belly) usually meant when we just say “bacon” without qualification. One can get Canadian bacon, cured side bacon, cured back bacon, uncured pork belly, uncured side meat, uncured back meat, turkey bacon, beef bacon, and several other things in bacon-like categories. If you just say “bacon”, though, you’re probably going to get pork belly meat cured with salt and/or some form of sugar, and possibly smoked.