Series 1: Gary Rush was guilty of the assault + break in; why fudge it?

The Gary Rush story was a bit daft when you think about it. He broke in the pensioner's flat, assaulted him and robbed him. Fine. However, instead of investigating properly, police decided it might be him, but probably not, but he needed to be put away for being a b@stard anyway. So they decided to frame him for the very crime that, coincidentally, he committed. They destroyed possible DNA that would have linked him unequivocally to the crime scene. They manipulated the mugshots in the hope that it would influence the victim (but the victim had in fact seem Gary Rush's eyes). All along they didn't need to do anything but their jobs, and all along, while they thought it was a longshot that Rush was the perpetrator and were bending the rules, it was him. I found this stupid.