I do not know how they interpret the ToS but this jumps out at me as possible things that could have gotten you flagged:

create more than one account per platform to access our Website;

use another person or entity’s email address in order to sign up to use our Website;

sell, transfer or try to sell or transfer an account with us or any part of an account including any virtual currency or virtual goods;

If you are using more than one account (or sharing several accounts among friends) then it's possible that they interpret that action as a ToS violation. They could spin it as transferring the account (even though you aren't selling it, you ARE transferring it to another person for free to use) or say that by managing your account (and at least one of your friends' accounts) that you effectively have two accounts. That's a very strict reading and interpretation of the rules, but it's not out of bounds for them.

Personally, I don't really see any harm in sharing accounts between people, but as the ToS is written, it does not appear to be permitted. The fact that you have never been previously flagged doesn't mean that it's not illegal. It just means that you've never been caught. Assuming you've ever driven a car, I'm willing to bet that you have exceeded the speed limit. Strictly speaking, every time you exceed the speed limit, you can potentially be flagged and given a fine. In the example of speeding, cops are usually OK with you driving too fast.... until they decide they are no longer okay with it. Driving 10 over? Ehhh.... no ticket. 15 over? That's a ticket. 10 over again? Ummm.... yeah.. ticket.

Many rules are loosely enforced, which makes enforcement seem arbitrary. I can fully understand you feeling frustrated. While they are completely within their rights to ban you, I would hope they would reverse the decision. It doesn't help them to turn away a paying customer, so they may decide to reinstate your account, provided you promise to no longer share accounts. The rule may be stupid, but sadly, they get to make the rules. And they get to enforce the rules, whether it's strictly or loosely or something in between.