I'm going to defend United and LIverpool here.
Of course disasters have knock on effects, and you can garner a lot of support as a result. I'm not going to say tragedies are a good thing, but clubs can benefit from them, as United did. They got world wide sympathy as a result, which certainly could only have helped them gain fans.
Liverpool's slightly different, since Hillsborough happened in the midst of a hooligan era, and in that specific case, they came a cropper as a result of reputation over reality.
If WE had a disaster, be honest, we'd be forever remembering it, and reminding others of it. There's even a thread on here for Foe. It's normal, and natural.
At the risk of offending many - there's a bit of a national culture of remembrance these days - remember this, and that, and mark the passing of this and that - outside of football. We'd be no different and would exhibit pretty much the same behaviour.
Liverpool is a bit more troublesome than United though - due to Heysel. It's just wrong to lump Hillsborough and Heysel together, but it's just as wrong to remember one whilst trying to diminish the memory of the other.