Reach's rain works by spawning "raindrop" particles into the game's particle buffer; to some degree, they exist as "real" objects in world-space. The raindrop-type particles are set up to create splashes when they collide with something.

Reach's GPU-driven particle system checks for collisions by comparing a particle's location to the z-buffer; when collisions happen, it can reflect the particle based on the surface normal at that location (and taking an elasticity value into account). If a particle's velocity dotted with the normal of a surface is positive (i.e. if the particle is approaching from behind the surface), the game's particle system supposedly (according to Reach's effects tech presentation) deals with this by removing the particle for being stupid, rather than let it do bizarre crap at the backface of a surface.
This is why I'm surprised, and might take a closer look. It would be one thing if the raindrops were falling through when I'm not directly looking at the platform's underside; they wouldn't experience collision if there's nothing in screen-space to collide with! But when the surface is in view, they shouldn't be falling through it. But what if the game is spawning the raindrops very close-by (i.e. on and around the underside of the platform).

It would be interesting to see the logic it uses to spawn raindrops. :)