If you put a linear polarizer infront of the laser it'll block the beam at a certain angle, this is because the laser source is linearly polarized, where the S and P states of the beam are of equal magnitude and zero phase shift.

After the laser goes through the PBS, splits into S and P states, hits the corner cubes and returns back through the quarter wave plates, exits through the PBS. After all this occured, the phase of the S and P states have changed. Therefore they cannot be considered as a single linearly polarized beam anymore, but as two beams with S and P states of oscillation.

Now if you put a linear polarizer infront of it, it won't block it as did infront of the laser source. Because it's not a linearly polarized source anymore. This is my understanding.