That always felt like a dubious standard. Parkland is a public good, but the Supreme Court would insta-smackdown any FD-style restriction on political speech in parks.
FD just required a media outlet to make sure the media basically gave another candidate the opportunity to access, it didn't actually require equal time, not unless that candidate wanted to, or in the case of advertisements was willing to buy them.
Certainly parkland should be equally available to members of different political parties, but it's not like you have to use the same standard either: the nature of a "public good" would determine the nature of what fair equal access meant. For airwaves it's one thing. For a limited physical resource it's another.