Salmonous, it seems pretty clear to me that you would need to ask directly for permission to use them, and it's a case-by-case, but otherwise it's a strict no. I'm not Nifflas but I'm thinking that's the case here. Also, I agree, it does take time to make tilesets of such quality- but I think the time required is worth it and I'd personally rather know that I spent a lot of time on my own tileset rather than simply borrow someone elses without them wanting me to. I personally don't blame Nifflas for such a policy. As a tileset creator, you don't want people to simply use your work in every level, when it's supposed to be unique to only yours. He's probably quite partial to them, and in his position, it would be hard to watch people trade in their creativity and time for his all the time. He already has plenty of tilesets you can use, so I don't think him being more strict on certain ones is unfair.
On the topic, I am glad Nifflas has revoked his statement on Saira, since it's quite difficult to come up with entirely original graphics for such a complicated graphics style. (lots of details, different objects, etc). I personally thank him for that, although I don't know if I will personally end up with a finished Saira level.