The implications of the "Out" threats

With the UK In/Out referendum less than three weeks away, and the BetFair odds on "Leave" starting to come down - albeit still very far from 50-50 - it has been instructive to listen to the veiled, and not so veiled, threats about what will happen if the populace vote for "Leave".

A good example was the comment in late May from Jean-Claude "Piss Artist" Juncker, European Commission President:

"The United Kingdom will have to accept being regarded as a third country, which won't be handled with kid gloves.
"If the British leave Europe, people will have to face the consequences -- we will have to, just as they will. It's not a threat but our relations will no longer be what they are today."

Apparently EU officials don't want to have lengthy negotiations[1] with a Brexited UK, which makes sense. But of course, the easiest course for both sides would be to retain status quo ante: continue trade under the same conditions and tariff schedule as before. Why wouldn't this be the starting point? In general, trade tariffs hurt the populace of the country / countries that impose them: they make imported goods more expensive for their populace. The main function of trade tariffs is to protect local industry from "abuse" from "dumping" by foreign manufacturers: selling goods below the cost of local produce. This may not be good for local industry, but it's certainly good for anyone who wants to buy those goods, at least in the short term.

It seems fairly clear that, whatever the merits of the "Remain" and "Leave" positions, the EU establishment is happy to cut off its population's noses to spite the UK's faces. One has to ask: if national membership of the EU is supposed to be of benefit to the population, why would the EU take action to screw over all their population in order to punish a member nation that wanted to leave?

[1] Note that the EU can apparently spare the manpower to negotiate a mostly free trade agreeement with Canada, which has half the population and a bit more than half the GDP of the UK.