The short answer is yes, the longer answer is that he died while fighting for union rights, not because he was fighting for union rights. Yesterday, Media Matters hit Glenn Beck for expressing disbelief at the idea that Martin Luther King Jr. supported unions:
Well, to make the point -- here's the deal -- April 4th is the 43rd anniversary of the day Martin Luther King was assassinated after speaking on behalf of the striking black garbage collectors in Memphis, Tennessee. So, I'm sure that the fact that they were black and in Memphis had nothing to do with his mention -- with his, uh, message. It was all about unions and collective bargaining. I'm sure that's what it was.
So, the interesting part of this, at least for me, is that Beck, in attempting to rationalize MLK's support for union rights, argues MLK only supported the sanitation workers because they were black. This is the guy who insisted last year that he was "reclaiming" MLK's legacy. This is the sort of cognitive dissonance that results when you try to divorce a single line from a single speech from the entirety of MLK's life of advocacy for economic and social justice in an attempt to appropriate and distort his message.
MLK undoubtedly supported the right of workers, even public workers, to organize, as evidenced by his support for the Memphis sanitation workers strike shortly before his death. However, Earl Ray shot MLK because Ray was a white supremacist who opposed integration, not because he hated unions. I actually think it's a little weird that unions sometimes try to blur the distinction, though not quite as weird as conservatives trying to pretend that King was some kind of small-government conservative.