Roadies in general tend to come off as having a bad attitude but I think it might be the circumstances. Usually these types when on thier bike are out for a hard workout. They are often trying to keep thier heart rate up or trying to improve thier average speed. This means they are busy working while on thier bike.
Commuters are often not pushing as hard and maybe just enjoying the ride or the scenery and so are often much more friendly. Familys also enjoying the MUPs also tend to fall into the more relaxed ride category.
Ofcourse there are always exceptions to these rules.
As for Roadies on MUPs. In some areas riding on a MUP at an off hour like first thing in the morning is often a good place to get a workout. There are often fewer intersections, little traffic and a pleasant course. Now those who try to ride fast on congested MUPs or who won't slow down for congestion are a problem but most riders are more considerate.

I am not a typical roadie but more closely resemble one than I resemble the guy on a MTB with cutoffs. I do ride fast on a MUP as part of my commute. However I give warning of anyone I meet, and most commutes I only see a few other users.
Craig