I think they crossed the line. Just wee bit. I mean, I'm not a rat lover or anything. But if kept clean, as in a pet, they are pretty damn cute. Smart too. Not as smart as my dog IMHO -- HEY! Let's wire up four dog brains next! Yeah, yeah, that's the ticket. How about a monkey? Why not!
These animals have a consciousness. You can't deny that. No, it is not at the human level, but a life none-the-less. How fucking freaky cruel is it to take a consciousness and tie it together with three others in some form to just see what happens? How freaked out were these rats in their little disembodied brains.
I'm guessing you didn't read the article, the monkeys have already been done:
Nicolelis published a second paper, also in Scientific Reports, describing a Brainet that allows three monkeys connected at the brain to control a virtual arm on screen across three axes.