I don't see it as theft, because I'm not getting something for less than the normal price. As an example, let's take the Harbor discussed earlier. The normal cost is 60 shields. Assuming we got twenty shields accumulated (I think he mentioned a forest chop and a few turns of travel time), the "bill" is 40 sheilds remaining. That would require 2 population at 20 shields each. For a pop-4 or higher city, that would be an easy rush, you would lose 2 citizens and the harbor is done. For a pop-2 or less city, it can't be done, as that would eliminate the city. The tricky case is the pop-3 city. The computer will initially say no, because it's too many citizens, more than half. However, by swapping to a Longbow (example) at 40 shields, and pop-rushing it, we now have 40 in the box, and a size 2 city. Swap back to the harbor, and rush it, costing one citizen for 20 shields, and you have your harbor and a size one city, with 2 "un-smiley's" worth of pop-rush unhappiness. You have paid the same amount (40 shields) with the same cost (2 pop, 2 unhappy), you just had to break the transaction into different parts.
Compared to your store example, I look at it as trying to buy something, and having your credit card declined as being over-limit (I'm sure it's never happened to anyone else, just me

). So, you then divide your purchase, paying for part of it with one card, then paying the rest with another credit card (or cash, or whatever). You still paid the full amount, the merchant still recieved their full value, you just had to use a roundabout (and annoying) method to complete the transaction, versus waiting for your credit limit to increase (or your population to grow to 4!)
And I do believe, that if the salesman (Firaxis) had wanted to call the cops (hard-coding in a restriction) they would have. As I said, I almost wish they would have, but they didn't, so that's the way the game is played. If you don't like it as a rule, then I wouldn't do it. There are many aspects of the game I have come to disagree with, or think are too 'cheesy', so I have chosen not to do them (ship-chaining, free palace jumps, etc), at least not in normal games. But that's my preference, it doesn't make it an exploit when someone else does, unless we had agreed beforehand not to use them.