Superman II Reconstructed Speculation
The time travel issue is addressed, and one suggestion mirrors some others here about cutting out the kiss. Since the earth-spinning bit was concocted pre-Back to the Future, I don't think writers at the time thought as much about plausability or common sense in regards to time travel "rules" and effects. It's probably safe to assume that before something like BTTF dealt with time travel in mainstream entertainment in a more logical manner (changes to the past rippling through the timeline, being erased from existence, etc.), it was viewed more literally ("he'll just reverse time and make her forget-- sounds good").
After reading that WB apparently did not compensate Donner for his time on the new version, maybe the earth-spinning was tacked on in lieu of a more clever ending if he wasn't getting paid and wanted to wrap it up. Who knows...