why does plants v zombies now require play station plus?

It always required PS plus to play multiplayer since day 1.

If you just have EA account, you can only play by yourself.

Probably you were talking "playing with friends for free" event,  3-4 weeks ago where you can invite your friends who doesn't have game to play multiplayer with you.

I'm not sure exactly how it works but I see a lof of new players during that event. 

Unless I am mistaken the fact that you need a Playstation Plus subscription to access multiplayer game play is not an "EA thing" but is actually a PSN requirement set by Sony on the PS4 system (but not on the previous PS3 system).

You can play in the backyard, Gardens & Graveyards and Solo play without Playstation Plus. Everything else requires it.

I have friends who have played and do play multiplayer  without playstation plus for a fact.

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.