That comment about sharing tweaked style presets was probably from me from a post several days ago.
The styles here are from presets from various Yamaha keyboards that users can download, that should be no legal issue. There are no 3rd party styles or tweaked 3rd party styles, that are currently sold elsewhere, shared here. I would never share 3rd party content, even if tweaked. To answer your question, copyright infringement would probably be a case by case situation as in any artistic dispute, rather than a specific set of existing rules, but I am not a lawyer.
What I meant in the post was a suggestion that I would like to see users share their tweaked/edited/newly created styles, based off the presets, not 3rd party content. If they are voluntarily shared that would create no legal issues. This is not what is offered here in the styles download section. As I said, those are all preset styles from different keyboards. What I am proposing, and this is really a suggestion for Yamaha, not this forum to implement, is something very similar to Yamaha’s Soundmondo resource, where synth users can upload and share their tweaked patches or even own creations from scratch as long as they are based off preset waveforms, not 3rd party waveforms. Or in the case of arrangers, based off preset styles. I hope that makes sense.