They can, but that's not how they like to think, good algorithm designers like to find general solutions to problems that would apply regardless of the underlying platform/plugin/tool. They know that whatever footprint a plugin is leaving can be hidden or changed. People don't currently hide them because google is not penalizing them, so from an algorithm design pov, it's a waste of time for them to go after every little plugin that does something nasty. Think how they went after affiliate links, and then now everyone is using gocodes or whatever, they can go after that as well, but it's a lost game. They would rather spend their time finding an intelligent solution that would deal with the general problem.