Do the toilets have some suction mechanism, for zero gravity conditions? And as another here noted, don’t they dump into space? Or is that not allowed for environmental reasons?
Just dropping it out the door means it has the same speed and therefore same orbit as the ISS. That's a lot of crap flying parallel to you. You need quite a change of speed for reentry. For example, the Space Shuttle required a change in velocity of 200 to 550 feet per second to have its orbit intersect with the atmosphere and start reentry (see Shuttle Crew Operations Manual). Even after Burrito Buffet Night I can't get above 100 feet per second. :-)
Because it’s zero gravity, the shit would merely float along side the ISS, eventually coating and breaking stuff on the outside.