That reason is copyright law...which, unless I'm mistaken, CBS doesn't control
Don't be ridiculous. CBS owns all the streaming rights to their shows. The problem is that they're trying to sell those rights.
By not streaming to, say, Canada, they create a product out of thin air that they can sell: Canadian streaming rights. As far as I know, the only network to buy this new "product" is Canada's Comedy Network, which purchased the streaming rights to a number of Comedy Central shows, including the Daily Show and the Colbert Report. So if you try to stream any Comedy Central shows from Canada you're redirected to The Comedy Network's (awful) website.
US networks have always sold their shows' broadcast rights to foreign TV networks, so it only makes sense that they'd try to do this on the web as well. Unfortunately, the implementation is terrible. For example, The Comedy Network bought the streaming rights to the Colbert Report so they could drive traffic to their site and run their own ads in the streams, but even though the content is legally available in Canada any blog links and embedded videos that point to Comedy Central's web site still won't work at all in Canada (they all redirect to the front page of the Comedy Network). It breaks the web.
I don't think regional streaming restrictions will be around for much longer. For all the money The Comedy Network spent on the streaming rights for the Daily Show and the Colbert Report, all they got was a bunch pissed off viewers who direct their rage at The Comedy Network instead of Comedy Central. I sincerely doubt any other networks will bother to buy streaming rights unless these serious implementation problems are sorted out.