Maybe it's that a boombox playing music is more social and open, whereas what someone does with their phone is typically meant to be private, but some people play their music or w/e noise out loud from it as if only they can hear their phone and you're just supposed to pretend they aren't overstepping boundaries.

I hate when people have boomboxes too

Yes. At least the person with a boom box brought quality speakers.

Also, their sick roller skate moves timed to their music are pretty great.

Edit: Plus, it's not inconsiderate, because everyone loves "Wham!" (Classic 1980s music, from the last time I saw a boom box in public.)