It's fine that it CAN be used online.
It's a deal breaker for me if it it (even once) HAVE to be.
It's a deal breaker if it artificially limits functionality, for example, by only providing dictionary functionality via an online service, or a substantially worse offline alternative.
It's a deal breaker if it continually nags me to connect it. It can ask once or twice, or show a "disconnected" status icon just fine, but it shouldn't nag, it shouldn't pop up dialogs reminding me to connect or otherwise hinder me in using it for it's primary purpose.
It should be able to fulfill it's primary purpose (for an ebook reader: provide a comfortable and convenient reading experience) should be 100% available without me needing to ever put it online.
Forced software update on first boot is an absolute deal-breaker. Any kind of enforced scheduled update is a deal breaker.
As for the motivation: Control, predictability, reliablity.
Control: I want to buy a product that is in some known state, and then myself decide if I want to risk an update (for an ebook reader that's so badly done that it needs a software update to do the one thing it's supposed to do, that's a nogo).
Predictability: I generally don't want my stuff to change, I like my stuff for what it is, and if I want it to change, I want it to be MY decision, and not something forced upon me.
Reliability: I prefer the bugs I know, and can work around today, to the bugs that I don't know, and must learn to work around tomorrow.
Reliability is more than simply uptime for me, it's also that I'm able to rely on exact behaviors, I want to be able to rely on the thing to behave the same way tomorrow and in 10 years, as it does today. I want to be able to learn how it works, and not have to throw away what I know at random times for no reason.