The thing is, even nurses who have no explicit desire to 'sabotage' an unmedicated labor have reasons to encourage medication. I'm a doula and I've attended about 15 births in a lot of different hospitals. So I've probably worked with about 30 or more different nurses, and there are some commonalities. I would only say about 1 or 2 of those nurses were actively hostile to me or my client's wishes at any point - but a lot of my clients, who are mostly "I'll wait and see if I need the epidural" sorts of people, have ended up highly medicated. As Engineer Mom said, it's very hard to resist it when it's being dangled in front of you.
IME, nurses have certain responsibilities which are easier for them to fulfill when a patient is medicated. First off, they're supposed to be monitoring the baby and contractions, either on a schedule or continuously. An unmedicated mama usually likes/needs to move around a lot, even if she's on the bed or staying in the room. Movement makes it difficult to get a continuous reading on the monitors. Medication makes it a lot easier to stay still. Second, there's often a 'schedule' which labor is 'supposed' to follow - which the doctor expects it to follow - which is much more likely to happen if a mama is on Pitocin. There are differences of opinion about whether Pitocin labor is harder than physiologically normal labor or not, and not having had a Pitocin labor myself I don't know which side to come down on. But a lot of women feel that they NEED pain medication if they're having Pitocin. It's hard for the nurses to be put in the middle between a mama whose labor isn't meeting that schedule and a doctor who expects it. Also, nurses are usually managing several labors at once, so overall, the more predictable and less demanding a labor is, the better for her.
So I can understand why a lot of nurses encourage medication of various kinds. I don't think it's ill intentioned on their parts. And I think most of it comes from the rituals of the institution more than any personal desire on the nurses' part. As a doula, I don't expect to walk in and change all the rituals of the institution when my client has chosen to be there and has been educated about what is likely to happen but doesn't have a strong will to do the birth her own way. When I do hospital births, I expect to make small differences in how my client experiences birth, to empower her to make any choices she is inclined to that go against the ritual (which is often something small, like eating and drinking when she wants to), to minimize trauma to whatever extent I can. I don't expect to make a homebirth happen in the hospital, and I don't promise to protect my clients from the hospital environment. This is just me - many doulas have a more activist approach, or a more directive approach. I can't do it those ways - for my own sanity - but I don't disrespect those approaches at all.