A recent article/interview in Polygon with various execs from SONY revealed the fact that free to play games have the option whether they're published inside PS+ or not.
Meaning that free2play on-line multiplayer games do not require an active PS+ subscription.
Shuhei Yoshida, president of Sony Computer Entertainment Worldwide Studios:
As far as free-to-play games are concerned, it's a publisher's decision whether they put it inside or outside of the PlayStation Plus requirements
Further, it's confirmed that titles like:
- DC Universe OnLine
- PlanetSide 2
- BlackLight: Retribution
will not require PS+ for on-line multiplayer.
Another title that was announced for PS4 did not make a stance on the issue, Gaijin's WarThunder.
I made a personal inquiry into this, if I get a reply I will let you know.