Any comparison between an ISP/Telecoms company and Twitter/Facebook/YouTube is fatally flawed on several levels.
Off the top of my head, and baring in mind it's nearly 4am.
One is the legal frameworks in which the operate.
With an ISP/Telecoms provider they have pretty much a blanket "common carrier" exemption and what you do with their service is basically private (they are not meant to monitor it in detail except where the law requires, usually with a warrant), they can't tell what you're doing with it, and in many instances there is also a universal obligation to provide it to whoever will pay for it as long as they don't break the law.
Youtube etc are mainly "public" , what you do on them is open to anyone to see, which gives the service provider greater responsibilities in regards to how it's used, although they do have a defence of "we were unaware of it before X". Youtube etc also have no responsibility or obligation to provide services to anyone, as long as they don't discriminate based on one of the legally protected groups (and being a CT selling tat isn't one of them).
So you've got one where what you say is done with the pretty much complete expectation of privacy from the provider, and one where you're basically putting what you're saying in public for the world to see, in the former the ISP/phone company won't have a clue what you're talking about even if it's a plan to murder someone, in the latter you might as well be shouting it in their front office.
Then there is the basic business model.
ISPs/Telecoms companies have you as their customer and the access as their product, you are paying for your use of the services thus it's not in the interest of the company to throw you off unless they have to (you start making harassing calls for example and they'll soon kick you off once the verified complaints start coming in), likewise if you are using the service in a manner that is detrimental to other customers, or if you start to cost them too much because you're breaking the terms of service (running a busy webserver on your home connection, or sending out spam email).
Twitter/Facebook/Youtube etc all have you as the Product, it's great for them if you get loads of subscribers/viewers as long as advertisers (who are their customers) can put ads in front of you.
However when you start to behave in a manner that annoys the advertisers and thus putting them off buying adverts on the service, the company has to do something (this is where a lot of the "unacceptable" content sections of their T&C comes from*, stuff that may be legal but drives their customers away), as they have a legal responsibility to try and do what is best for their shareholders.
Now if you're using their service to spew nonsense that is actively aimed at causing offence that's going to put off advertisers, when you are doing that and aiming it at victims of terrorist attacks and mass murders, that becomes a PR nightmare for the advertisers whose ads may be playing between parts of the videoclip or alongside the written content.
At this point any sensible company that is aware of it's obligations (forget even basic human decency) will start thinking about how such a user is putting their financial situation at risk, and if enough advertising is affected they have a choice of "allowing free speech" and breaching the duty they have to their investors and shareholders, or getting rid of the account and keeping the money flowing.
Given the size of some of the advertisers it might only take one or two major brands (Proctor and Gamble for example**) to get cold feet about their products being promoted alongside someone who is calling the grieving parents of murdered primary school children liars and encouraging his braindead followers to harass them, and you've got a situation where the company is likely to take action - lose one user that is on the edge of the T&C and has gone over it at least once, or lose an advertising account worth millions.
In short.
ISPs etc you are the customer and what you are doing is private and no one elses business as you're paying for the use of it, Youtube etc is public and you are being allowed to use whilst someone else is paying for it.
*And why youtube demonetised a bunch of categories of videos, and why for example we don't allow a lot of stuff on these boards (as the boards reflect on the shop, so some stuff is straight out).
**Given much of their products are aimed at families, it probably doesn't sit well with their brand to have advertising playing alongside someone who is spewing vitriol at grieving families.
