Obviously Nvidia would include DLSS in an AMD sponsored title as it would show you are better of playing it with an Nvidia card as DLSS > FSR. Obviously AMD block it as they don't want that happening.
Neither of those things is obvious.
I don't think that for most gamers having access to either DLSS or FSR is a game changer. That's what the poll on this forum showed.
So I would guess most developers are happy to include the tech in their games if Nvidia and AMD helps them to do it. All Nvidia has to do to discourage developers from being sponsored by AMD is to refuse to help them implement DLSS.
The better question would be what incentive is AMD offering for developers to be AMD sponsored that makes them accept their sponsorship? AMD has to offer more for the developer than having no sponsorship because without sponsorship Nvidia would help them implement DLSS (presumably).
As for why Nvidia doesn't implement DLSS in AMD sponsored titles, that's a decision Nvidia is making not AMD. They probably want developers to go with them because DLSS is better than FSR without any other incentive. That's the cheapest option for them. If they break their stance and just add DLSS anyways they lose the ability to pressure developers not to accept sponsorships for added revenue.
AMD has the worse offering at the moment; they need to kick in more to make it worthwhile for developers to go with them. However, they also don't need to commit more resources to try to make FSR better than DLSS. This probably makes more sense for them as Nvidia already has hardware in their graphics cards that makes DLSS function better than FSR.
I just don't like the presumed conclusion that this thread is jumping to that AMD is being anticompetitive and forcing developers to not implement DLSS when they accept an AMD sponsorship. Both companies have enough reason to do what they are doing without either engaging in anticompetitive activities.