You weren't there. Clearly.
You're used to dialing with ISDN or DSL and have it connect?
Know what you got 90% of the time when you dialed up with a modem back then? A busy signal.
That's cause for every modem that existed, 5 guys wanted to use it. This went on until cable and dsl, late 90s or so.
So it was considered rude to dial up, then go away and leave it connected when you weren't using it and people were waiting.
Free ISP's (there were many, Barry was just the first pay-for one) would jut disconnect you and fuck you that's why. And most didn't charge so they could do this. But if you were paying, yupi might not expect that o had to be aware of the rules, like don't spam and don't tie up the modem pool.
You don't like it? Open your own isp and you can do what you like and waste your very expensive phone lines on people that forgot to hang up when they went out.