They're legal for muzzeloader season so far as I know. The only requirements are that it be percussion or flintlock, .45cal or above, use 50+grs of black powder or equivalent for deer and that it not be a pistol.
You probably don't see the bolt actions because they're just an unneccesary design that's darn cumbersome relative to the original straight pull "short bolt" inlines as well as the newer break action inlines.
There was no need for the additional complexity of the long bolt and striker system. The bolt muzzleloaders already have a breechblock screwed into the rear of the barrel to contain pressure, so the large bolt and striker assembly becomes redundant. The same job is easily done with a short straight pull bolt and striker assembly,rather than one that is long enough to be a centerfire rifle bolt and requires standard bolt manipulation. I think a lot of folks realized that,so we don't see too many of them compared to traditional inlines.