Was the developer doing something "outside what they should be allowed to"? No. It's their package. I would argue that intentionally introducing a breaking chan

Was the developer doing something "outside what they should be allowed to"? No. It's their package. I would argue that intentionally introducing a breaking change (I would count "go into endless loop" as "no longer performing the same semantic operation") should also be coupled with a "bump major version". Was the developer being an arse? Yes, definitely. Technically and morally correct are two very different things. And doing this sort of thing does have an impact on your reputation I suspect that the dev now have limited at least a few possibly-lucrative jobs down the line.