It is planned to make riot enable encryption by default when it is fully stable but I totally understand why the protocol would allow it to be disabled. Matrix is about more than instant messaging and there are possible use cases where you may be using it for something that needs no e2e. Maybe you are using it to control the lights in your house and you want to write an SQL script that can read the commands sent to your light over matrix and tell you how long they were turned on for last month. You can't do that if the commands were e2e encrypted and you gain nothing by encrypting them.