Perhaps it has nothing to do with huffing paint. Perhaps it has nothing to do with a "Nanny State". Perhaps it has *everything* to do with the fact that we can't have nice things anymore.
Rather than taking pot-shot guesses as to why this is, I simply asked the department manager when I was in one of their stores. The answer:
They got tired of having to replace the fixtures every year because they were caked with paint and looked like a Jackson Pollock masterpiece. As a chain, they take pride in their appearance, and having a paint-splattered area in the model section was outside corporate appearance standards. Funny thing, too--this same thing didn't happen where the large spray paint cans were displayed with the other craft paint. The only area of the store where paint was on display that was abused in this manner was by the plastic models.
In other words, it was done because some people make it impossible for all of us to have nice things.