The higher-cost Chromebook 550 (Intel) does support 3G.
So your entire argument is based upon a misunderstanding, is it not?
However, my perspective is that Chromebooks are meant to be used with a connection to the internet, which makes the 3G model sort of mandatory (for 3G/LTE access is more accessible - in my experience - in developing countries than full internet connections). It's in that perspective that I hold this argument.
For the case I'm basing this opinion on, a basic internet connection (DSL) in Vietnam costs much more money than a basic 3G/LTE internet connection. Also, a DSL connection will only be usable from home. I know this DSL/3G relation is also the case in other developing countries.
One imagines some sort of tethering to be feasible, if not universally applicable, for those who want a computer form factor and already have 3G Internet on their phone.
[1] That is, over and above the 20$/mo for the phone line itself. So if I were to get a line just for 3G usage(so as to put it in a tablet or chrombook, for example) it'd be more like 30$/mo.
So this $250 + ~$600-700 over 2 years for a total of almost $1000. I'd consider that instead of the "low sticker price" when comparing options.
It's the 330$ 3g model that's got the deceptive pricing.