I just had to reinstall Firefox Mobile (clearing all settings) and then I wasn't able to zoom on a lot of websites. Fortunately FF has an option for that but I

I just had to reinstall Firefox Mobile (clearing all settings) and then I wasn't able to zoom on a lot of websites. Fortunately FF has an option for that but I wonder why there needs to be such an option. Why would a website-maker decide it's better when I'm not able to zoom on their website and why can they do that at all?