Yeah, the reason for that is most of their owners are conservatives who didn't want political statements they disagree with.
And this isn't to suggest there aren't conservative American soccer players (Geoff Cameron, hello!) or supporters. There are.
Just as we saw last year with the flareup last year between @ISCSupporters members and MLS teams, don't be surprised when MLS teams - despite the league putting on a happy, we support you face - quietly oppose much of the protests we're seeing in sports right now.