This topic has probably been posted before. With Comcast data caps, I am hitting the limit quite often. The main reason is that my elderly aunt doesn't remember to hit the home button before the power button in order to stop the streaming. This causes, in my estimate, about 6 hours of unnecessary streaming (2 hours during the day and 4 hours when she goes to bed).
I would love to see an enhancement to Roku sticks to either 1] automatically go "Home" when the power button is pressed or 2] add an option to stop streaming after 1 hour of inactivity (in addition to the 4 hour option that already exists).
I agree that the power button should do more than just turn the TV off. It's one of the more confusing things users don't seem to understand. There are other options if your Roku device is powered via a USB power adapter and your TV has a USB port and that USB port can supply at least 1 Amp. Most TVs turn off the USB port when the TV is turned off, so that would power off the Roku. They also make power strips with the feature that when the main device is shut off it powers off the other outlets in the power strip. The disadvantage to either of those is that you have to wait for the Roku to start up every time you turn on the TV and since the Roku is powered on all the time it may not be able to check for updates on a regular basis.
As for the suggest for other options other than 4 hours, there's a place for suggestions: