We really like to create mutual benefit with these internships. The students aren't free, so we want to get some actual necessary labor out of them. On the other hand, we want to turn them on to our branch of science. Most undergrads aren't exposed to the work we do, even in the broadest sense, so we feel it's really important to show them some cool stuff to make their experience good.
But. Research isn't fun
It gets tricky to design a good experience for undergraduate research participants. We want to get a bunch of work done while exposing people to the things we find so exciting and at the same time, not create unrealistic expectations for their futures should they decide to pursue research careers. I have seen several bloggers (but now I can't remember which ones) write about how they had a cool undergrad research experience but were then somewhat disillusioned by the bullshit when they got to grad school. I can see how it happens, since it's pretty easy to shelter an intern from the bullshit for 10 weeks and an internship can have a huge impact on a student's career choices.
So: handling interns is tricky. Our ethical consideration is usually we can't have them just wash dishes. But maybe there is also an ethical issue with making an internship too good for the intern. Wouldn't want to give anyone false hope, now would we?