If McDonald's genuinely thought Supermac's expansion would cause customer confusion, then they're not only right to challenge it but they have a duty to challenge it in order to protect their trademark.
Which brought Lord Godfrey MacDonald, of Clan MacDonald, down on them (“I am The MacDonald”).
Such behaviour is prohibited most places. That these companies try intimidating people with spurious law suits makes it easy to dislike them.
My own personal favourite is a dodgy gym eqipment company which has been recently bullying various blogers for using the generic term for a group of cyclists, peloton. They can’t have the word it belongs to common language.
It’s crazy. It’s like if McDonalds tried to trademark “hamburger” and then threatened to sue people who live in Hamburg.
What's particularly annoying is that their product is a spinning "bike" which has nothing to do with road cycling apart from pedalling. Specifically the Digital Peloton Vlog, which reviews cycling simulators and software.
So no, McDonalds does not have a duty to do so, only an option.