Have I offended my professor by applying to summer research at other universities? Yesterday I applied to a bunch of internships and sent my professor a request for a recommendation letter. He denied me the request, and sent my request email, along with his reply, to everyone in the lab. He wrote that he didn't understand why I would apply for a research internship in (our research area) when I am already a research intern at his lab, and that he assumes that I was only here for a reference letter and I have no intention of contributing to the lab.
This was shocking to me for several reasons. He had previously wrote, 2 months ago, that he was happy to write me a recommendation letter. In addition, I had already applied to a research internship in (our research area) at a national lab and he provided me a recommendation for that. On top of this, I have been working with a grad student with in-person lab work 4 days a week. I always show up on time to the group meetings but I usually don't have much to share since the grad student I work with share our results.
I also thought that it was obvious I would apply to research internships in our research area, and that it was common for undergrads to work on summer research elsewhere. I don't understand why I would apply to internships in other research areas that are not related to our lab. On top of all this, these REUs are funded and I applied because I can get paid over the summer. My position at the lab is a volunteer position.
Was this an offensive request? I'm not sure why he felt so strongly about this since he has always been very nice.
Edit: Thank you for all your suggestions. I emailed him explaining my decision and he sent the group an email saying that he values my contribution and supports my application.
Tell him that you do intend to participate as previously agreed, but that funding for the summer was an important consideration. If he is at all reasonable, then that should be enough. If he is offended, then he isn't being reasonable.
If he offers you summer funding, then consider that, of course, but he should have little controlling say over a volunteer.
Perhaps he values your work so much that he thinks losing you for the summer would be a blow. Alternatively, he is just a jerk. If he hasn't got a reasonable response, you should find a way to be done with him. What he did is unprofessional, probably unethical.