No need to opt in! And why should there be? Well, when mobile phone networks have offers, they seem to quite often require you to send a text or go to a website or whatever to enable your cheap calls or texts or whatever. Sometimes you're allowed choose between a default charging scheme and an alternative which is better in all ways, for example. Why do they do this? I'm not quite sure.
Amazon do something similar with their Associates (affiliate) programme. Originally, affiliates got a flat 5% of most items sold. After a number of changes, a scheme whereby the more stuff you sold the higher your percentage cut was came in; you started at say 5%, and could go up to 10%. Then they did something weird. They created two schemes; a default scheme and a 'performance' one. Initially, the default one would work better for people who only sold a few items per quarter. After a while, though, the 'performance' scheme was better in all ways. But it wasn't the default, and you would have to visit the site to change to it. This wasn't even made very clear on the site; you had to hunt around. At least for a while, it would reset to default at the end of each quarter. The whole thing was very strange.
So, why do they do it? Well, Amazon may have some incentive; it'll encourage affiliates to visit the site periodically, and see what new affiliate tools are available and so on, thus boosting earnings for Amazon. But I can't really see any reason that a phone company would do it, unless they want to appear to be giving out a great offer but know that only X% of their customers will bother to enable it.