Licensing is the key to business. The truth is, SouthParkStudios.com, the way I know and remember it, can be streamed around the world... at least that's how I remember it because I was able to watch SouthParkStudios.com from Hong Kong.
Anyway, back to the point. Comedy Central/MTV Networks has to make episodes "clear to stream" to the site, so there's a pre-set time of which they allow anyone to watch these episodes on the internet. The reasons as to this are not disclosed to the public, but the least they have done is to give a clear sign of when to expect the episode to be available unless otherwise noted.
Some episodes may not be available at all, but knowing that there's a pre-existing contract means it will be online unless pulled by the said undisclosed contract.
The long story short is, they had it planned all along and there MAY be other people (like those probably outside the U.S.) that haven't seen the episode yet.