The respected PBS program above can call itself anything it wants. But a freelance editing gig had Mr. Rewrite looking for AP's style on frontline/front-line/front line. So here you go:
It's front line as a noun: Mr. Rewrite works is on the front line (or on the front lines) of the war against improper usage.
It's front-line as an adjective: Mr. Rewrite is a front-line warrior against improper usage.
This is an exception from Webster's New World College Dictionary, which lists frontline and front-line for the adjective.
There are many references to frontline among articles indexed by Google News. And many of those are by organizations that Mr. Rewrite suspects pledge allegiance to the AP Stylebook. He understands how folks might tend to think frontline as an adjective (see longtime). He has less patience (and Mr. Rewrite has very little patience to begin with) for those who go with one word as the noun.