Proprietary software licensing has such a bad reputation because it's an inherently monopolistic power ripe for abuse:
- a proprietary vendor's copyright license over their software is protected by the full coercive power of the regional government. In the specific case of the US, this includes the ridiculous statutory penalties for copyright infringement.
- a vendor's promise to not abuse that power generally can't be trusted, as "doing the right thing by our customers" is not typically a legally binding obligation, and the history of proprietary software is a history of dubious practices by monopolists.
- Open source licensing (even on a time delay from the proprietary version) is a powerful way for a vendor to make a promise to do the right thing by their customers in a legally binding fashion.