history

It seems to me that there really was insufficient grounds for accessing these emergency powers. I wonder why the others in the parliament didn't question whether it was just cause or not.

The law is only as strong as those willing to enforce it. Germany was in ruins after WWI, and the preceding democratic socialist government had only made things worse with their economic policies, so presumably most Germans in the ruling class were willing to look the other way to let the new government clean up the mess.

Plus, it wasn't complete propaganda that communists wanted to overthrow the German government. For years, communist groups in Germany had been committing acts of violence against their political rivals, including the Nazis. That violence actually helped the Nazis assume power in elections, since they advertised themselves as bringers of law and order, where the communists wanted to burn everything down.

Whether the Reichstag fire was an act of arson by actual communists, or a really believable false flag, we'll never know for sure.