first of all: if someone runs a service they have the right to define how that service operates. This may include fees, access methods etc.

secondly, as pointed above the java applet may be placed in a page with information or advertising that represents some dollar value directly or indirectly. IRC access thus is charged to match.

I can only assume you get user name and a password to access that system, otherwise any one could enter. This means they have some administrative costs to mange this, It also means their network is unlikely to have floodbots entering or any other nuisaince connections.

mimiking java is possible as you know, but not all java applets are equal. The cost of the service is very low if it provides quality chat environment.

in the end I think the charge is for IRC access not mIRC access, which just happens to be the most popular IRC client.