Why can I not perform a fast forward push?
Git has an option known as fast-forward updates (sometimes referred to as a force push). By default, this option is disabled in Beanstalk as it can have unintended consequences for those newer to Git. A worst case scenario could result in data loss as your repo history can be overwritten.
However, if you're familiar with how the feature works, it can be enabled under Repo > Settings in your Beanstalk account. Please note this is a repo specific setting.
For more details on fast-forwards, review the Git documentation.