Mostly because no one was buy their US version of Cadbury's (it's nothing like actual Cadbury's, which is why most Brits looked to import it, we grew up on Cadbury's, so we like what we know, Hershey's version is not that), so if they take away the version that everyone wants access to, they're left with nothing but the Hershey's version to buy (well, or do without). It makes sense from a business perspective, because people were circumventing their patent, but it's still a move they didn't really need to make, expats still aren't going for the Hershey's Cadbury's version, they're just paying a bit more to get it from home now...