I think it's a bit more than a culture clash. The issue is not only that the majority of source code readable online is open source; it's that making code readable is the only thing you need to do to allow people to copy and modify it. It's a bit like a failed Searlian speech act where an actor running lines or a student practicing French is overheard and presumed to be saying something to someone in particular. The OP's readers, having looked at his post and code, quite understandably mis-inferred that he meant the posted material to be beneficial or useful to his readers, when really he only intends it to be useful to himself in some hypothetical future where he wants a job.

To my mind, this is similar to saying "look but don't touch" about a door handle, an elevator, or a platter of hors d'oeuvres in a crowded party. Of course people are bound to be confused; that's not the way to go about bragging to people about your fragile antique doorknob, guerilla marketing for the Otis elevator corporation or demonstrating what a really well made (prototype) bruschetta looks like.

This isn't merely a cultural issue, where we're used to the availability of code being a signal that it's open source. It's a deeper issue of social cognition than that. Other people are seeing the hatetris post, and making the perfectly normal inferences that humans as sophisticated social creatures are bound to do -- namely that if he's putting stuff up publicly and even points to it enthusiastically, and that stuff offers natural interactions to the reader, then it must be because he intends his readers to partake of those natural interactions with that publicly available stuff. Here the stuff is code and the affordances are reading, copying and changing, but I they needn't be for this error to occur. The OP's just failed to consider that his readership might mistake his exhibiting of untouchable code as an invitation to interact with it.