If I was the manager and someone came in to complain about some guy like that in the parking lot, I'd tell them to put their cart in the goddamn corral. 
And that's all well and good, but most store managers would not do that, for several reasons. And as has already been mentioned in the thread (and keeps getting ignored), most store managers don't care whether or not customers put carts in the corral and don't see it as an issue. Most of them do see it as a problem if their customers (rightly or wrongly) feel threatened by rude people and talk about taking their business elsewhere.
And the bottom line is, there's just no good reason to be rude to people.
Well, I was sort of joking if you didn't catch it because there's no way I would ever be a grocery store manager in the first place.

However, I disagree with your theory. That really isn't the bottom line. There's a lot more to it than people's feelings supposedly being hurt or threatened somehow because someone calls them out (or points something out). You're right about one thing. Most store managers probably wouldn't do that because they are too politically correct to care and just don't have the time anyway.
My take is this, which has been mentioned previously in this thread (and keeps getting ignored). 1). It isn't rude to remind people that the corrals are there for a reason. If they don't like being reminded and it is a source of tension or embarrassment for them, I don't give a shit and don't think anyone else does either. 2). The company pays to have the corrals built as a central collection point so their employees aren't wandering around the parking lot all day collecting stray carts (I've mentioned this before). That costs the company even more money, hence the reason for the corrals. 3). The cost of product increases as a result of corrals not being properly utilized. 4). Increased prices in the store is what causes people to take their business elsewhere. Not some random guy approaching customers in the parking lot. How often does that happen anyway?
Hell, it's probably mostly other customers doing the harassing. You know, the ones who actually return carts to their rightful place? All of these things have been mentioned over and over. So, where does the "people's feelings getting hurt" part come in? Nobody cares about that. It's not about being "rude". It's about people being called out on their bullshit.
If grocery store management wanted to pay their employees to walk around the parking lot all day collecting carts, why spend the extra money providing corrals? This is all very simple business strategy and is designed to benefit everyone involved. People not returning carts is what I would consider to be rude and thoughtless. People who just can't seem to be bothered with taking an extra minute or two (tops) to contribute to the cause. To do their part so to speak. After all, the carts are provided to customers as a service and convenience. The least they could do is return the favor (cart).