There are several reasons why people would do this, but the decision is often not made by the person writing the code, so it may be a red flag to you, but it doesn't point to inexperience of the author.

In addition to the MSVC issue mentioned in the sibling comment, I've worked on projects that used clang thread safety analysis, which at the time (and maybe still today) was only available for C++, so the build for that analysis was configured to build all C source files as C++.