It makes their job harder since they not only have to clean a clog but also do it at risk of coming into contact with dangerous chemicals. Also a lot of people are a bit too liberal with their use of chemical clog removers and that can damage piping and cause leaks, especially in the case where you use them but the clog does not clear. Now on one hand for a plumber that's a good thing, because it's expensive to repair, but it's a lot of work, and it's difficult, and people often give plumbers the stink eye for asking for a lot of money without recognizing the fact that they caused the damage.