Take it from someone who worked for the police: What they did is illegal, period. Cops don't do this kind of thing because once it comes out (and it inevitably does) it destroys their case against the defendant.
You may be thinking of cops lying to a suspect during an interrogation, which they are allowed to do, but at that point the suspect has the opportunity to invoke his Miranda rights/5th amendment right to not incriminate himself, as he is being detained and questioned and knows the cops are who they say they are. When initially confronted by cops disguised as service workers asking for entry to his house, he has no way to know that they are cops and therefore cannot legally consent to any evidence gathering.
You may be thinking of cops lying to a suspect during an interrogation, which they are allowed to do, but at that point the suspect has the opportunity to invoke his Miranda rights/5th amendment right to not incriminate himself, as he is being detained and questioned and knows the cops are who they say they are. When initially confronted by cops disguised as service workers asking for entry to his house, he has no way to know that they are cops and therefore cannot legally consent to any evidence gathering.