You're correct that people are giving them jobs, but for every job given to them - that's one less for US citizens, especially older teens who should be doing a lot of these under-the-table jobs. They're still taking those, regardless of whether it was offered or not. To answer your question though, yes some businesses are encouraging the problem which is already illegal, but they do it anyways. I suspect that some of them (Businessmen) do it with good intentions, but most I assume are just willing to bend/break the rules to save a few extra bucks.
And you're correct that not all immigrants, legal or illegal are Hispanic, or Mexican to be more particular, but a huge percentage are. The latest trend, as far as I am aware, is actually Chinese and S.E. Asian illegal immigration. No more, or less problematic, but definitely still a problem.