So this isn't a bug in mythbuntu, it just so happens that plymouth is pulling in the mythbuntu package. If it doesn't want people to pull in the mythbuntu theme, it should Recommends plymouth-theme-ubuntu-logo | plymouth-theme instead.
Also, any of the metas that aren't pulling in an explicity plymouth theme need to start doing so.
So this isn't a bug in mythbuntu, it just so happens that plymouth is pulling in the mythbuntu package. If it doesn't want people to pull in the mythbuntu theme, it should Recommends plymouth- theme-ubuntu- logo | plymouth-theme instead.
Also, any of the metas that aren't pulling in an explicity plymouth theme need to start doing so.