I mean, I was good with people hanging up during the spiel. Saying "sorry I'm not interested" is more than fine and likely to put you on the upper end of calls that's day. Swearing or getting upset isn't acceptable and fails to recognise that neither of you want to be on that call.
However telemarketing has a better response rate than paper based surveys, and the data is used to drive decision making. A lot of people complain when things don't work out for you - like local council decisions on new amenities, but if you don't submit your opinion you can't be heard. It's not perfect but it is used to define a surprising number of things.
Wrt to you MIL, yeah, it's not nice to be on the recieving end of hostility, I get it can be annoying but as I implied above nobody chooses the job, I personally had to take whatever I could get and I was too young/not out of work long enough for JSA (plus JSA is a pain to get anyway, but that's a whole different story).
You ended your post with "fuck telemarketers", on a post where I was highlighting that I hated having to do that job, so yeah, you demonstrate a distinct lack of empathy, if there's a wider context on that comment please let me know, as I'm not trying to strawman you, when you explicitly stated I deserved to be treated like that.
The fact you think telemarketing is about selling a product and not market research (about all sorts of things including political opinions, or NGOs etc) shows that you don't know what you're talking about and seem like an angry person. I'm going to disengage now, but I hope you can find peace :)