Alright, alright. I guess it was offensive.I still don't see how it is though. That's all.
It's just the essence of what you said. Though part of their work is cleaning up after the students, you made it out as if their entire livelihood was based in garbage. The fact you made that joke in the first place is also rude as it only works with a job that is generally viewed as lowbrow or demeaning. You wouldn't walk up to some screenplay writer who was collaborating with a friend and say "Why are you asking him about it? That's what they have media executives for!"
Rule of thumb, do not demean people for their job.