My incentives are never going to be in perfect alignment with my employer: I work for them to build my own wealth, and they employ me to build theirs. The foundation of the employer-employee relationship is that each provides the other with a means to realize their incentive in a way that's mutually satisfactory, if asymmetric. If this relationship is out of balance, either party is free to engage in negotiations or terminate the relationship.

Employers have concrete strategies available to them for maintaining their position in this negotiated relationship. Typically they will have a performance review process, which should have no problem determining whether they're getting what they're paying me for. Separately, the legal system we're both subject to protects legitimate intellectual property perfectly well without overbearing noncompetes and invention assignments; cases of physical property theft, etc., are also covered.

So what's the problem? Why should employers have additional rights to make sure that, if the situation is ever remotely muddy, it goes their way by default?

(I say should in the moral sense, and the broader sense of socioeconomic utility. In a might-makes-right frame, obviously they should have it simply because they are strong enough to take it. That is what we have right now, in practice.)