They might call this "caching" but it's still a memory leak. Over time, the application consumes increasing amounts of a finite global resource while not appreciably advancing its own progress, and negatively affecting the whole system. That's a memory leak.

The solution is for operating systems to inject deliberate back pressure on memory allocations for user-selectable applications to stop them from behaving this way. "No memory for you. Try mallocing smaller chunk. Maybe I grant; maybe I don't. Maybe I just kill -9 you if you bug me too much." This is exactly how I wish my OS would treat web browsers.