IRC servers don't have to prevent filesharing, many don't. Some prevent it anyway either because they'd prefer not to risk legal action or because of the stigma of DoS attacks that is often associated with filesharing (largely untrue), or possibly because they actually believe IRC should be for chatting. In any case, there is no legal requirement or precedent on IRC servers and filesharing.
Bottom line: Some IRC networks stop filesharing, some don't. I highly doubt that there will ever be a point when all networks agree on the situation.
