What can be said for fast food is that, for all the franchises and chains that exist, there are only but a few options on offer, and that the distinction is in the differentiation. In that way, fast food isn't all that different from any other industry; many make similar products, and where they get the edge is in their own "special sauce" that puts them above the rest, be it process or branding or small but distinct differences. It's that "special sauce" that needs to be defended, and the franchise that gave us "special sauce" itself is looking to maintain its hold on branding built over decades.
McDonald's needs no introduction for any American, but it's likely necessary to provide some background on the Australian fast food chain it's suing for trademark infringement. Vice offers a brief but sufficiently comprehensive overview of the Hungry Jack chain that now rivals McDonald's Down Under: when Burger King expanded to Australia, their name was already taken, so the chain paired with its subsidiary to open locations in the country. A legal battle later resulted in Hungry Jack's taking full ownership of all the franchises across the continent, leaving it as McDonald's top rival — the Australian Burger King to Australian McDonald's.
Rivalry is all well and good, but when supposed copying is introduced, things inevitably turn litigious. McDonald's is suing Hungry Jack's over its trademark applications for the Big Jack and Mega Jack sandwiches, which it claims infringes upon their own Big Mac and Mega Mac marks. While Hungry Jack's claims that they applied for the marks and were met with no objections, McDonald's claims that it did object, and that Hungry Jack's moved ahead regardless.
Thus Hungry Jack's committed a cardinal sin — not in creating a similar sandwich, but in so blatantly thumbing their nose at McDonald's with a derivative name. No restaurant-goer is so naïve as to think that the combination of hamburger patty, bun, lettuce, tomato and some mixture of condiments is entirely novel to that chain, but we all understand that there are those differences, and that those names indicate to us exactly what we're getting. Hungry Jack's might argue that no consumer would expect to get a McDonald's burger at one of their locations, but the name might suggest that they're getting something quite similar; in that, they may have overstepped the mark, as far as legal liability is concerned.