This article is from the NYPD Blue FAQ, by Dave Chapman, Alan Sepinwall and Jeff Knapp with numerous contributions by others.
This one comes up a lot, though, ironically, it's Steven
Bochco's own fault that viewers complain so much about it. On
Hill Street Blues, every single suspect brought into the
station was quickly rushed into the protection of noble public
defender Joyce Davenport, leading all TV cop show fans to
assume that all suspects automatically made use of their
Miranda rights to counsel.
According to NYPD Blue producer and former real-life NYPD
homicide detective Bill Clark (see 2.10.), in real life,
suspects understand their rights, but often choose not to
exercise them out of naive optimism. They figure that if they
ask for a lawyer, they'll be stuck in the legal system (and
therefore in lock-up) for at least 48 hours, and if they
weren't being looked at hard as a suspect before, the cops are
really going to bear down on them now. If, on the other hand,
they try to speak for themselves, they figure maybe they can
come up with some kind of plausible excuse/alibi and get away
Whether Clark is exaggerating the truth a bit in order to make
the show seem more plausible is unknown, but that's their
story and they're sticking to it.