I can see your point, mjw - my best guess is those names reflect the club owners (Real, Chivas are both names of clubs in Spain and Mexico as well), although most clubs in MLS use more standard names like Seattle Sounders, Portland Timbers, etc.
There seems to be a real Euro- and Latin- envy among US soccer, and the names show that to some extent. The British tendency to use singular nouns to describe a group of things, in our case a team, is reflected in LA Galaxy, Houston Dynamo, New England Revolution (a play on the Patriots, by the way), Chicago Fire, etc.
Again, I can see how the names would come across as pretentious - they are more reflective of already-established soccer cultures around the world than they are of the existing tendency in American sports to name a club [city/state] [plural nickname]. Stanford Cardinal is a notable exception, which also doesn't help the "pretentious" impression.