Did a little research on Pennsylvania's primary process. It is closed.
But that only means that Crooks had to mislead the political registrars into thinking he was a Republican. They didn't know his intentions, and because we as a country cannot impose tests on someone in order to vote, or to associate, they aren't required to know what his intentions were for registering as a Republican.
In this case, it was to vote for another member of the Republican party primary not named Trump. While open primaries allow for crossovers which makes it easy to skew the results of a primary, closed ones are just as easily influenced by someone simply lying about what they are on a registration form. There's no crime for lying on your political registration form.
The very mechanism that allows a person's views to evolve from one party to the other (allowing them to scrap their registration and reregister as a member of the party that most closely represents them) allows for someone to directly for someone to register for the party that represents a direct antithesis to their actual political viewpoints to exploit the process and skew the final results.
I admit I overlooked that. But the type only changes the tactic. It doesn't mean Crooks was a full-blooded Republican, but in name only.