This is exceptionally irritating when the browser is more than capable of displaying the full site properly, and the mobile site doesn't have all the features of the regular site.
Continuing a bit off topic on my rant, another terrible UX is finding an article you want to read in a search, tapping the link, and finding out that article isn't available in the mobile site, and simultaneously not giving an option for the full site. Taking it even further, some sites force you to download the mobile app, and only then do you find out the article isn't available in that app! I'm going to stop now - I'm getting so mad just thinking about it.