It's worse than just illegal content. Copyright doesn't allow you to redistribute anything without the permission of the copyright holder. IPFS however has no means to track the author or the license of content.

That means even distributing a piece of perfectly legal Open Source becomes illegal. Unlike a tarball or even a torrent where you can bundle content and license, IPFS allows addressing individual files or blocks, thus stripping the license from the content, which most licenses forbid. This does not even require an intentional action on the user, but happens automatically by partial content landing in your cache.