I have a few possible explanations for that:
1. The T500 was created for Sony/GT, and has a precursor USB security chip which is sufficient for Sony to be able to guarantee it's a wheel not a hack tool. So Sony allowed the wheel to work with full FFB and button capability in game. [Of course, if Sony was interested in this approach, why would they not allow people to use their old DS3 as a secondary controller in game?]
2. Basic HID support is not actually governed by the security chip atall, the only thing the chip protects against is use in the PS4 menus where a USB device could be used as a hack tool. And because TM has licensed the T300 they have fully FFB-capable drivers inside the PS4 OS, which can be piggy-backed by the T500.
Either way, the only chance I see for native full FFB Logitech support is for Logitech to want to license, but since Logitech is officially out of the console peripherals game and G27 definitely doesn't have any kind of USB security chip, I fail to see either potential scenario being valid.