It's all just a scam to get people to buy new iPhones. If your 3GS is Jailbroken, it's possible to install it and play it anyway:
The easiest way is to install an App called IPA Installer. After you install the game, edit the Info.plist file inside the PvZ 2.app to remove the line pertaining to the front-facing-camera requirement which should read "iPhone required device capabilities" (see pic). Delete the whole entry and save the file. Just tried this two days ago and confirm it works with the current version of Plants vs. Zombies (2.8) on an iPhone 3GS.
