[plorkyeran]

The place where it makes sense for a library to read environment variables is where the program is not written to use that specific library. For example, I can link a program whose author has never heard of TCMalloc against TCMalloc rather than the system malloc, and then configure TCMalloc via environment variables. This does not require modifying a single line of code, while manually forwarding configuration onto the allocator would. Another common example is configuring sanitizers. Not having to do anything other than pass another command-line switch to the compiler is one of the things that makes them really painless to use.

I do think you'd be hard-pressed to find a situation where a program calling setenv() to configure a library actually makes sense. It's a pretty strong sign that someone made a bad decision. People will, however, make mistakes in API design.