The Whole attempt to Trademark the Word won't happen They've taken a few videos down already but the likelihood of the Trademark being accepted is pretty low. Right now the internet has 30 days to file a counterclaim, which a large group of youtubers/streamers are doing.
Its a dumb thing to do on theFineBros behalf.
But that being said, the Finebros are Trademarking the Genre, They Are Strictly Trade Marking the Word.
"There is an entire genre of Reaction videos online and an unlimited number of ways to make this type of content. We do not hold a copyright on reaction videos overall. No one can. " (http://www.finebrosent.com/reactworldfaq/)
What there doing is Trademarking specfic FineBros Brands such as:
Kids React, Teens React, Adults React, Elders React, People vs. Food, People vs. Technology, Try Not To Smile or Laugh, Lyric Breakdown, and Do They Know It.