Do professors in the STEM fields at U.S. universities get summers off?
While no one is technically forcing you to do work over the summer, you'd be foolish to take extended breaks. 

One of the benefits of a tenured faculty position at a research university is that you get great flexibility in your schedule. Today, I left work early (e.g., 4pm) to pick up my kids at summer camp and go to the park. I have no (or almost no) formal obligations over the summer.

That said, this evening, I was doing data processing and reading through papers. It's now 1 AM.

My long answer to your question is that it's certainly possible for a research active faculty member to take a whole summer off, collect summer salary, and pick up in the fall with research and teaching.

This sounds like a very bad idea.

Right now, funding levels are so competitive, that everyone I know with research funding is trying to be as productive as possible throughout the year. The summer might be a bit more laid-back, and I bet most research faculty spend less time in the office or lab. Most will probably travel to at least one conference or workshop, and I bet most also take a week or two of "real vacation" with family or friends.

But, if you're not working on papers or grants year-round, you're probably falling behind and will likely have difficulty getting that grant (and summer salary) renewed.

This summer, I helped submit a major center grant, I'm drafting another grant for my group's research, and I'm trying to finish up ~3 papers. Add in some papers and grant proposals to review as service. I'm not trying to boast - I suspect this isn't an unusual summer work load for research-active faculty.