DOC Governor Inslee has been a pretty cool head through all of this. He's assembled a good scientific team to advise him. (In case you're wondering he's a Democrat, I'm a non-Trump Republican). He's done a good job so far.
The 250 number is based largely on crowed control and keeping some space between people. There is some evidence that the possibility of transmission is lessend if you are more than six feet away from a person with the virus. I'm sure there was some consideration of the idea that if you banned everything, people would ignore the ban. Yes its arbitrary, but there's some thought behind it.