That's the whole point. If you build something on top of open source code, your code should be open source too.

Even if this is the case I might not want my library or application to be copyleft. Or even if I do - I might not want everyone else in perpetuity who uses my code to have to use a copyleft licence.

This goes back to the fact that not everyone can choose to use a GPL licence and in a world of compromise and collaboration, that can be a blocker.

Those of us who disagree are happy to see our software used in any context in exchange for attribution.

The problem that occurred in this case is someone at Microsoft taking the code without following the license at all.