I just tried a sanity-check. I went from having Wi-Fi turned off, clicked the ad at the bottom of the main screen, and Wi-Fi automatically turned itself on. I wonder if, in the rearranging process, I got ahead of the device and clicked where an ad was going to appear... I'm going to contact Amazon Kindle support and find out if this is intended behavior (either purposely or accidentally click on an ad, Wi-Fi automatically comes on).
Yes, as Betsy posted at just about the same time, this is designed behavior.

So you'll have to train yourself not to touch that section of the screen when you're on the home page.
You might want to consider a cover which will help with that.
OTOH, the WiFi really doesn't cause much extra in the way of battery drain. . . .definitely faster than not having it on, but, for example, if it's got a fairly full charge, it's not going to drain in a couple of hours while it's in sleep mode and you come back to find it basically dead.