If your car was sputtering they oil change might have been one of the first things they did, to make sure it wasn’t an issue with oil pickup or old oil sludging up the valves or creating to much resistance in the engine, Shops don’t make any money on Oil Changes so if they did it they probably had a reason to do it, I’m also sure that the paper you signed had a line in it saying it’s okay for them to do “fluid changes if deemed necessary” unless they’re charging you $150+ for it I’d just pay it and thank them for looking after my car