I'd say that the range of types that females on TV are allowed to play is fairly narrow, and so we are used to narrowing our definitions of the extremes to "slightly more girly" and "slightly less girly". And I do think they feel compelled to remind us that their female police officers are female by having the long hair and stylish clothing.
(and AE is correct that the choices in the original article are odd and slight, I'd add under researched. They might be onto something, but they haven't exactly proven their point)
A possible exception that proves the rule:
Detective Shakima Griggs (an out lesbian character) on the Wire. Now I've still not seen The Wire (it's on the DVDs to obtain list) so I'm mostly going from pictures online. Did anyone watch it? Does she get femmed over time? I know The Wire is at the very gritty end of the police drama scale. She still has long hair, but it looks like she's allowed to tie it back for work. Maybe they even gave her sensible shoes.
Looking at all the long flowing shampoo ad locks on tv detectives is reminding me why Olivia Benson earned the name Butchy McFabulous back in the day.
Oh, and another datapoint, from the Brit end: