Because Big Tomato runs a configuration of WordPress called 'MultiSite', which is a network of connected websites, site administrators cannot add Javascript or iFrames to pages/posts. This is a security matter because allowing for site admins to add unfiltered code is a big risk of breaking the whole network.
We get many benefits from using a network configuration, including the ability to share plugins, security measures, user accounts, content, and other elements, which makes us more efficient and keeps our pricing low.
If you need a script added, please email the code and the page where it should go to support@namieasysite.com and we'll add it for you. There is no additional charge for this service; it's included with your subscription. We're happy to add scripts for you!