"Your carrier may charge for SMS messages used to activate iMessage"

And?

The message is just a warning. Depending on what carrier you are using, you may or may not have to pay. Very likely not, unless you carrier chooses to charge for it.

Apple has absolutely no control over this, as its entirely up to the carrier in use whether the sms message to activate iMessage is charged or not. There are many many carriers, that will not charge for the activation SMS.

If you are using a carrier that charges for this activation message, Apple cannot in any way force them to not charge for it.

You can try changing carriers, to one that will not charge you for it.

If there are no supported carriers where you are, or you refuse to switch to a supported carrier, then yes, you’re going to pay every time you use iMessage. If you don’t want that, then don’t use iMessage with an unsupported carrier.

Apple does not provide all its various services everywhere, and some carriers are not participants with Apple’s various services. For example, Apple does not do business at all in Iran so there are no supported Apple services or products there.

Wireless carrier support and features for iPhone in the United States and Canada - Apple Support