why does plants v zombies now require play station plus?

Somebody might have PS Plus account and make that PS4 as his/her primary console.

All accounts that play on that console will get PS Plus benefit  (play multiplayer) for free, as if they have PS4 Plus.

Here is the details:

http://manuals.playstation.net/document/en/ps4/settings/activation.html

This should clarify your question.

Note:

I used this method as well. I only have 1 PS Plus account and 1 PS4. All other accounts on that PS4 can play PVZ2 multiplayer.