Bronies React falls under a video that parodies Fine Bros. React, just like how Cinemare Sins parodies CinemaSins. I'm pretty sure if Fine Bros. wanted to take down Bronies React, they'd have to go through a lot of legal loopholes to do so, and even then I'm not sure. No doubt it'd hurt the channel a lot, and I'm sure we'd all feel the effects of it, but you can't just say "We did it first, so it's our copyright!".
That'd be like saying the first band to play jazz could 10 years later make a copyright on all jazz music. It's not "ripping off our format", it's a genre of videos.
Besides that, wouldn't that make Let's Play videos "So-n-so reacts to this video game" videos?
Oh, and by the way...
*snuggles Cinni*