Target is a very big company that sells lots of stuff. However, they do not actually earn a lot of money. Their profit margin, last time I checked, was about 4%. I don't know what Target pays for a GE, but let's say it's half MSRP, or $75. So, before expenses, free shipping, etc., they make $75 if you buy one at $150. In reality, because they have lots of fixed and marginal costs, their profit is probably more like $10 or $20.
But instead, let's say you buy a GE at $250 because you can't get them at MSRP anywhere. Target surely does track competitor's prices, and it may also track an item's wholesale availability (or lack thereof) from Lego. In any case, if you buy the GE for $250 (and since they go in and out of stock at Target.com, some people surely do), Target gets that additional $100. They get all of it (minus a low single digits percentage lost to VISA or MasterCard) as profit, because there are no additional expenses -- they still have just the one set to pack and ship, etc. So Target has made as much from selling one GE at $250 as they made selling 10 or 20 at $150. In the case of some other items mentioned, such as clothing or (especially) food, the difference is even greater.
What I'm saying is that extra $100 of revenue from sale of a marked-up $250 GE contains as much profit as perhaps $1000 worth of MSRP-priced lego sets, or literally $10,000 worth of food (where Target's margin is more like 1%). I used to own Target stock and pore over their earnings reports, but anyone interested can confirm all of this by requesting a prospectus from the company (ok, anyone REALLY interested). The point being that this extra revenue from Target's "lego investing" has an outsized impact on the company's profits, and I would expect to see more of it in the future, and even Wal-Mart and Amazon joining in under similar circumstances (I would also expect the same to become standard practice in dealing with any other toy that appreciates in value post-EOL). I think we'll increasingly find that when only one online retailer stocks an item, unless that retailer is LEGO Shop at Home, we won't be able to rely on still obtaining sets at MSRP.