AFAIK those packages are kernel-specific tools or other applications that (for some reason) don't work on one or the other kernel (yet), but there is no reason why sysvinit would not work on linux (as long as somebody keeps maintaining it on linux).
The point is: once you add support for sysvinit to a package that runs on both kernels (which you have to do anyway, as Debian kFreeBSD requires it), you have already done the work to support two init systems, and there is no point in *removing* it again. (Maybe you could say that one init system is the default, and that others are supported on a best-efforts basis, but doing *extra work* to remove something that already works and doesn't break anything else is silly.)
Also, you didn't address my other point: if it were forbidden to add alternative init systems to Debian (and/or other distros), then right now systemd or upstart would likely not be in Debian at all.
And it would also not be possible for the actual users to compare which one works best in practice (taking into account a lot more real world use cases than any Debian maintainer or upstream developers can test).