The difference is that most U.S. sports teams names are proper nouns in the plural form (49ers, Raiders, etc.), but most motor sports teams are usually referred to as Team <Singlular Noun>. "Team Quaker State is in the lead." If they were the Quaker States, then it would be "Quaker States are in the lead."
I don't follow MLS, but looking at the team names, they mostly look singular to me, apart from "New York Red Bulls". I suspect the Red Bulls are nearly always referred to in the plural by Americans.
I don't follow MLS, but looking at the team names, they mostly look singular to me, apart from "New York Red Bulls". I suspect the Red Bulls are nearly always referred to in the plural by Americans.