Thanks for your contribution. It is frustrating when that sort of thing happens, especially if no mention was made of that possibility in the contract — be that a document, a verbal agreement, or what the legal people call an “implication of consent.”
Still, if you sell all rights in the work, as is most often the case these days with online copy sold to businesses and marketing ventures, then you have granted permission to the editor to do whatever she wishes with your copy once she’s paid you for it. And that’s the end of that. Pocket the tin and move along.
But if you retain rights in the work, then the specifics of the editorial remit you grant for the rights offered should be clearly stated, if not in a contract, then in the publishers guidelines — to which you imply agreement by submitting work.
I started out — as you did , if I recall correctly— before the online economy was “a thing.” SEO didn’t come into it back then, and any sub editor absolutely had the right to meddle with titles, and a lot more besides. But if a publication mistreats you, so long as you’ve been paid, I’d say just chalk it up to experience, blacklist the client, and go to greener pastures. Life’s too short, and the opportunities so numerous, it rarely makes sense to fight your corner in these cases.