...because it's all you're getting for free.
I'd like to point out that using the excuse "we didn't explicitly state this plugin does that" is not acceptable; It's natural to assume that any form plugin will include the ability to edit the form. You should explicitly state that this plugin doesn't allow you to customize the form for free so that people like me don't have to waste their time with this plugin and form a negative disposition towards your organization.
Also, the alert "* Note: The Form Builder is read-only in this version." doesn't tell me that you want me that I have to pay for this feature, it tells me I have a permissions issue on my server.