It will kill some roots and doesn't kill the trees. Trees send out roots in all directions for water. Problem is that the copper sulfate must contact the roots and only kills the ones they contact. Foaming stuff is a little better. Water flowing through the sewer pipe only covers the portion about an inch deep in 6" sewer pipe. I have ours with a manhole hole about halfway so we can run the rooter every other year. With all the faucets open it never runs more than an inch deep otherwise it just trickles down. That leaves all the roots in the top of the pipe that don't contact the copper sulfate and you still have a mass of roots in the pipe above the water level.
Even is the pipe is partially or totally plugged, the stuff never gets down to the full length of the plug to get all the roots. It may finally open up a slight channel under the roots and then it doesn't kill any more cause it needs to contact the roots for a length of time.
It is expensive but one recommendation I've seen is to just pour some on the ground above the line about a foot wide so it leaches down and keeps the roots from getting close to the pipe. Another thing is to keep your trees near the line well watered on the opposite side to encourage roots on the opposite side and discourage roots going towards the sewer line.