I disagree that strangers as a potential threat is one of the driving motivators for this practice. It may be a rationalization for it, but it is not a natural position to take.
In sparsely populated areas, where strangers are less frequent, strangers are often assumed to be friendly. People are still wary, of course, because there is always the possibility that a stranger is dangerous, but this is not a particularly strong reason to avoid them completely. The occasions where this seems to not be true are when individuals want nothing to do with anybody new, regardless of who they are or where they come from or what potential threat they might be (i.e. the old man with the "trespassers shot on sight" signs posted doesn't want anything to do with anybody).
In sparsely populated areas people will often take random strangers in need into their home for a night or two, far more than any small request the average stranger in a city might make, yet the people in the sparsely populated areas don't seem particularly put off by this.
Your second point I think hits closer to the mark. People believe it is rude to say no, and so seek to avoid situations where they can be trapped into small requests like you mentioned. Instead of learning to say "no, sorry", or "I'm sorry but this is a private conversation", we ostracize those who are friendly (a really sad state of affairs common to any large-ish city).
But I simply hang up. I say "no thanks", and the telemarketer goes on to the corresponding point in his script, and I simply hang up on him while he's in the middle of a sentence. That's rude.
This I disagree that this is rude. It is not rude for you to hang up on him after you decline his offer outright and he disregards you. That is extremely rude of him to do so. He is required to continue based on the nature of his job, but it is still rude. Taking his rudeness onto yourself is wrong.
It may be polite to oblige small requests, but it is not particularly impolite to decline them. You are not beholden to strangers, and there is nothing in the rules of etiquette to make you so.
This I think is also one of the major problems with people using a cell phone in inappropriate places. People seem to think it is rude to not answer the phone, even when it would be incredibly rude to those around you to do so. It's a conflict and most people seem to choose the caller on the cell phone for some bizarre reason, even when it isn't likely to be any kind of emergency.
This I disagree that this is rude.
You are talking about what ought to be. I am describing what is - how people think and behave. What we can observe is that many people have a great deal of difficulty getting off the phone when a telemarketer calls. The reason, I think, is clear: they are reluctant to end the conversation unless the other person lets them go, because this is conversational etiquette. That's why it's difficult. You saying that it ought not be difficult isn't a description, it's an exhortation. You're talking in exhortatory/advisory mode ...
I am beginning to suspect that it is surprisingly common for intelligent, competent adults to somehow make it through the world for a few decades while missing some ordinary skill, like mailing a physical letter, folding a fitted sheet, depositing a check, or reading a bus schedule. Since these tasks are often presented atomically - or, worse, embedded implicitly into other instructions - and it is often possible to get around the need for them, this ignorance is not self-correcting. One can Google "how to deposit a check" and similar phrases, but the sorts of instructions that crop up are often misleading, rely on entangled and potentially similarly-deficient knowledge to be understandable, or are not so much instructions as they are tips and tricks and warnings for people who already know the basic procedure. Asking other people is more effective because they can respond to requests for clarification (and physically pointing at stuff is useful too), but embarrassing, since lacking these skills as an adult is stigmatized. (They are rarely even considered skills by people who have had them for a while.)
This seems like a bad situation. And - if I am correct and gaps like these are common - then it is something of a collective action problem to handle gap-filling without undue social drama. Supposedly, we're good at collective action problems, us rationalists, right? So I propose a thread for the purpose here, with the stipulation that all replies to gap announcements are to be constructive attempts at conveying the relevant procedural knowledge. No asking "how did you manage to be X years old without knowing that?" - if the gap-haver wishes to volunteer the information, that is fine, but asking is to be considered poor form.
(And yes, I have one. It's this: how in the world do people go about the supposedly atomic action of investing in the stock market? Here I am, sitting at my computer, and suppose I want a share of Apple - there isn't a button that says "Buy Our Stock" on their website. There goes my one idea. Where do I go and what do I do there?)