When you send an email, you want to be able to select the sending address. I get that. And you CAN.
You cannot send without a sending address. The automatic setting can and does cause that sending address to change under your nose - that is a problem.
If you choose one default address, you regain control.
Important Notes:
1) You are NOT forced to use that address.
Press Command-N to start a message, and you can use the From popup to choose any of your addresses.
This is what you wanted. How else would it work, anyway? You can't have an empty "From"
2) The above DOES NOT affect a reply. Replies will come from whichever account the message was addressed to. So suppose you set Account A as default; a message is received for Account B; a reply to that message will have Account B as "From".