Under a legal system in which acquitting the defendant, or dropping charges after he has been imprisoned for some time, results in sizable cash penalties against the police department or its individual officers, the police have a strong incentive to repress their doubts and push for a conviction. How serious this problem is depends on a variety of factors. If there is a substantial chance that the conviction of an innocent will eventually be discovered and reversed, a police department that suppresses such evidence risks having to pay for years in jail instead of months. If, on the other hand, such a reversal is unlikely, suppressing evidence may be an attractive gamble. Another factor, of course, is how honest the police are.
Few people like to believe that they are personally responsible for putting an innocent man in jail.
Unfortunately most of us, including, I suspect, most police officers, are fairly good at bending our beliefs to fit our interests. Conclusions are left as an exercise for the reader.