Seriously, it's just a minority fringe element talking out their asses.

What do you think they'll do, redirect a rocket that was going to stop a space
rocket headed towards Earth? Distract some NASA employees at a critical moment?

I remember dealing with some anti-nuke types who used some really strong yet
unsurprisingly ignorant rhetoric to stop Cassini's launch a decade ago. You can
see how much affect they had. In fact I don't recall much of any noise when
New Horizons was lofted in early 2006, and that's got RTGs, too.

There are plenty of nut jobs on this planet who would like to ruin humanity in one
form or another. The anti-NEO stoppage crowd is among the least of ones to
worry about.