It makes a lot more sense when you learn that the rippling puddle effect isn't actually the wormhole itself. The wormhole is microscopic, the Stargate is more like a transporter pad that breaks you down and squirts you through it to reassemble on the other side. So there's actually an Ancient computer doing all the work, and it can "decide" how some of those rules work in an arbitrary manner.

Which does make me wonder why the heck the Ancients designed the Stargate to disassemble stuff that enters the puddle in a situation where it's got nowhere to send it. I guess that was all long before OSHA.