Why? What does unlimited SMSs have to do with it? (In NL unlimited SMS is pretty common anyway.)

In American billing scheme, the receiver and the sender both pay.

For SMS? And people put up with that? Here, SMS is free for the recipient, and quite often also free for the sender.

But even if SMS costs money, why not just use free apps like Whatsapp or Signal?

And if SMS does cost money for the recipient, does that mean it doesn't get used for 2FA? I imagine some people would block it in order to prevent unexpected costs.

because most of the people there use iMessage instead of Whatsapp or other apps like that, so if you dont have an iPhone you pay.

Wait, so people in the US don't use Whatsapp but iMessage, and using iMessage on Android is possible but costs money?

I had not expected either of those, but I'd imagine if iMessage costs money on Android, that would massively increase the popularity of either Whatsapp or Signal.