ext2 is not proprietary to Linux
You have to distinguish between a file system format, and an implementation of the format. The Linux implementations of ext2, 3, 4 are GPL'ed, but the format itself is completely open and anyone is free to write their own implementations under any license they want. For example, there are non-GPL'ed implementations of ext2 available for Windows, Macs, and some BSDs, as well as small read-only implementations in things like bootloaders.
exFAT is very different in that various features of the format are protected by patents (in non-free countries), and by missing documentation. MS's aim is that you cannot implement exFAT without licensing their code.