At least with the kindle, when I bought the ad-supported version because I wanted to save money, I just reset the firmware after unboxing and strictly put the books on via USB. I have no need for the device to be online and never once have been greeted by an ad. In Germany we have "Buchpreisbindung" which pretty much disallows eBooks to be significantly cheaper than physical books, so even with no ads shown, big A still gets their fair share of revenue out of me. I guess for smart phones and similar this is not a feasible approach because they pretty much turn stupid without internet access, but at least rooting on Android puts some leverage back into consumer's hands for now.