you have to explicitly permit use of your intellectual property. Publishing work and not providing that license means you retain all copyright, and if anyone uses your IP, they are breaking the law. you must specifically grant them permission, otherwise they open themselves up to a lawsuit at a later date, because they have no permission to use the code.

I do however strongly disagree that "code without a license is not open source". publicly visible source code is extremely useful for research and education purposes, and I have in fact published code that is "free to view, research and experiment with" but contains no license and specifically states that you are not allowed to use the code. I still consider that code open source, but it is of course not "free" (as in speech) software. Please do not conflate the two.