Is there a reason the offset of this opamp is done like this?
The input (presumably) and the output (definitely) are referenced to GND. By low-pass filtering whatever is connected to R63 and the 1.5V reference any noise on the 1.5V reference is also filtered and the impedance to ground is minimized. Many reference sources are relatively noisy and they may have relatively high output impedance at higher frequencies, far more than a typical capacitor.
It would be a rare situation where there was a reason to do this any other way.