Like many things, there are a lot of reasons why people do what they do. Some of them yes, are just mean and nasty. Most of them are just trying to do their jobs though. And any job in the prison industry you are going to become jaded and cynical at. Consider the fact that a fair bit of the people they have to deal with are not the 'I did my time and learned my lesson. Now I want to live a good life.' A best their clients (is that right term here?) are just as jaded and cynical as they are, and they know the system is stacked against them. They resent that they have to keep reporting in, justify any and every action they take while hopefully trying to keep a job, if they can find one in the first place. Most people in the parole office and their clients look at each other as adversaries. That never makes for a good relationship. Especially when one side has the ability to strip away the other side's freedom with a phone call and a signature. And a lot of times it won't be questioned. And that's not counting the fact that the system is set up so basically, you can be sent back to prison at any time. It's almost impossible not to break parole in some way, even if you are trying very hard not to.
In the best cases, the parole officer knows their clients are lying to them repeatedly. It's up to them to figure out when and how they are lying, and if it's something worth sending them back to jail over. In the worst cases, the parole officer if a corrupt scumbag who enjoys feeling power over their client's lives, possibly even extorting them to do things or threaten to send them back to jail. Since most of the time a judge is going to believe someone involved in law enforcement over a criminal, it's hard to stop them. Just all part of why the prison system is horrible.