There's an old saying in ethics circles: just because you can do something, doesn't mean you should do it. Being able to do something is simply a matter of acquiring and using the necessary expertise; knowing when you should do something relies on your own ability to make ethical decisions.
So, you think you can ˜do PR for a biker gang? For a military dictatorship? For a crime boss? For a tobacco company? I have no doubt that a well-qualified public relations practitioner would be entirely capable of applying those qualifications to the PR problems and opportunities of just about any client. The question is: is it ethical? And one other thing: what does selecting a questionable client do for your professional reputation?