Why Hosting Companies charge for subdomains ?

Why Hosting Companies charge for subdomains

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Adding multiple and infinite sub-domains is a matter of:

add what I marked in bold at httpd.conf
and to add a wildcard in the DNS.

<VirtualHost *>
ServerName www.example.com
ServerAlias example.com *.example.com


What are the reasons, that maybe I'm ignoring, so that Hosting Companies charge for subdomains or limit the subdomain to a fixed number ?
Control Panels limitations ?
Something else ?

I'm speaking of a situation where
httppoint.example.com/httppoint.gif and example.com/httppoint.gif point to the same file on the server so basically the configuration is like the one I posted before.
So once done is done forever without having to hack the server again...or rebooting apache...

Money people who think that it acutally costs more will pay it, also depending on the control panel but I would think money is the main factor, more than can get the better right?