I don't think this is anything unique to Celtics fans, unfortunately. Lots of people don't know how to properly show respect or observe basic social etiquette any more. That pretty much extends to every aspect of society currently, from the clothes people wear in certain situations to the tone of our political discourse.
MLK died trying to make the world a better place, a goal he accomplished in large part. He's one of America's heroes, and I don't think it's too much to ask that people take a minute out of their day to remember his accomplishments. I'm sure the fan acted out of ignorance (or inebriation) rather than malice, but I do wish people would be be a bit more respectful at times.