> It's quite strange to me that people would identify as or look for a "[language] programmer".
I understand people doing this.
First you can be more sure of what you're getting. It's sad that tests like FizzBuzz are so useful, but it's a fact. If I hire a Java developer for a Java position, I can figure out there Java skills. If I look at a PHP developer, it's more of a crapshoot. They may have a lot of PHP experience, but can they convert that to Java or have they just been doing cargo-cult stuff? This would be a smaller issue with something like C# that's closer to Java.
Second, and I suspect more common, is time investment. It might take someone quite a bit of time to switch languages. If they haven't done it before, you might find out it's a weakness for them. In my experience, at least in smaller companies, the fact that you're hiring means you need someone now. That extra time could be killer, because you waited to long to start hiring.
I would at least expect candidates to look into what we're using. When I changed jobs 2 years ago, one place I applied was a Python shop. I've never used Python professionally, but I've tinkered with it. If you're applying for a Java position, I expect you to have at least looked at Java before the interview. Sadly, I bet that would happen a non-trivial number of times.