have you TALKED to them about it yet? maybe they don't like them anymore either and might like the chance to remove them? I'd ask them first then decide on your course of action.
Yes, we've casually mentioned it to them. They like their trees and on their property, it looks lovely lining their driveway. They are close to 15yr old trees so I understand them not wanting to remove them.
They understand the suckering problem because they have it on the other side of their driveway too. The only difference is that for them, it is an established garden area so minimal traffic or digging to disturb the soil.
On our side, we would have several hundred of these little suckers right now. As a PP mentioned, these things have sharp thorns (think rose thorns on steriods) and they grow quickly. There was one that I was breaking off the shoots above the ground. The hard knobbly stump base was an absolute PITA to eventually dig out.
the other people are just intolerant, exaggerating what they would do
I don't think these people are exaggerating. One was the arborist who had the tree poison on hand and offered to do the suckers for free. My concern is that poisoning the suckers could kill the trees and it would be pretty obvious what happened.
I have no problems with poisoning the fish tail ferns that come over onto our side of the fence. Nothing seems to kill them and it's just a maintenance issue to keep them subdued.