When you disable form and then set focus on form, or any of fields, you can still edit that focused field.
I've viewed some source code and it looks like focus checks if field is disabled but do not check if the field is a part of a form and if that form is disabled.
BTW it should also check for 'fieldset' view because 'fieldset' also have enable/disable functions.
So you can have:
form -> fields
form -> fieldset -> fields
Disabling of form or fieldset should be respected down to fields.