Ethical dilemma implies that the police officers have discretion. If they have no discretion, they will have no dilemmas. It is true that police officers sometimes make mistakes, but on balance they are carefully trained in the law, procedure and ethical obligations. The procedures are designed to reduce the need to make ethical decisions, but procedures cannot anticipate everything. No two situations are alike, and police officers are still human and need to make quick decisions.
I agree that police officers must be allowed to practice discretion in some cases. It is the responsibility of the people doing the hiring and training to make certain that they hire professional people that will use discretion wisely.
While reducing police discretion may sound good on the surface, it would simply lead to the courts getting overburdened and the police's relationship with the public likely getting worse.
If police were required to arrest and bring people in (for example) whenever they had suspicions, the prosecutors would end up being overloaded with cases, many of which will just be a waste of time. At the same time, the people will likely be angry at the police as the police arrest people instead of simply talking to them or warning them. The same sort of thing would happen if police had no discretion as to when to issue traffic tickets.
Basically, there is a need for discretion whenever people are assigned to enforce rules. Society has to give the discretion to whoever enforces the rules and then they have to try to ensure that the people are properly trained and will do their jobs. But discretion can't be taken away.