If we handled their cocks in the same rough manner they'd soon be screaming and moaning... and not in pleasure!
You see I don't necessarily agree with that - having seen some of them wank so hard it looks like they're going to rip it off! I think the rough handling is possibly a bit of a projection because that's how they like being touched.
Another point is that when people are very horny they often enjoy much rougher handling/shagging than when they're not/being paid to join in and look interested but are just trying to remember what time the washer is going to be finished so they can get the next lot on. When you have one of each in the same room then there's going to be a disconnect, and I suspect part of the problem is that they either forget or don't want to remember that the only person in the room for the sex is them.
unfortunately most of them dislike being told to stop and are offended when u show them how to use their fingers.
Provided they're being told calmly and politely, offending the sensibilities of somebody who thinks it's OK to cause pain, discomfort and potentially injury is not something I can get worked up about - if their ego is so fragile that it can't cope with being told their technique is hurting me then it, and they, can fuck off.
Thankfully the vast majority in my experience don't want to do this and actually listen. The ones who don't don't get to come back.