"Is there some silly law prohibiting drain plugs? "
Supposedly YES although I've never seen it cited. It is also apparently considered bad engineering practice to put in a drain plug, which chould vibrate out or strip out or corrode out, releasing the critical engine oil and stopping the engine. And, going into the bilge and then the water outside.
With no drain plug that just can't happen.
With a drain plug plus a catch pan plus a safety wire it can't happen either...but no plug at all is the cheapest simplest surest way to make sure it can't happen. Supposedly required by 'code' somewhere, somehow.