Many free assets on Itch have no license attached. In most cases, from reading the comments, it seems like these authors generally follow a CC-BY approach (when asked, they only require credit), but without an explicit license each person wanting to use the pack will have to get the author's explicit permission. I suspect that in most cases, this is simply because most authors don't realize what not including a license means. 

Itch chooses to not force authors to pick a license (fine since some use their own), but can there be a warning displayed when uploading assets with no license?

A possible solution could be to force the user to pick a license, including the possibility to pick "his own (attached  or linked)" or "CC0".

Asset creators can't be forced to pick a license because not all of them might want a blanket license, let alone a free culture one. Maybe if there were options like "custom (see inside)", and even then a warning would be better. Assuming creators don't know what they're doing is bad, and forcing them into choices they might not like is worse.

I agree that forcing them to pick a free culture license is the wrong approach, but forcing them to in some way acknowledge that their choice is no licence rather than merely leaving one out by inaction (and providing some guidance on what the choices mean) would help a lot. That said, my original suggestion was for a warning for just such reasons.

BTW, it's especially bad for for-sale assets. You really have no idea what you're actually buying. That opaque zip file could theoretically include some non-commercial license making your purchase worthless.

I never suggested defaulting to a permissive license.

I just suggested to force them to pick a license, any license, including the possibility of uploading their own customised personal license.

Just force them to acknowledge that there must be *A* license, otherwise they could just not upload anything, because it would be worthless anyway.