I'd say that it's deeper than that: in my opinion, people who divide life into personal and professional are missing a large chunk, which could be called as Public, or Social life. And there is necessarily an overlap between that and professional life, because most people work in a social organization (like companies) or at least interact with clients.

And so I think the issue is that, while a political opinion can be considered part of the private realm, it's not unfounded to consider a contribution to a political campaign - which has a direct impact in society - as part of the public realm, and hence open to scrutiny.