No third party tax company allows you to CANCEL a tax return once it has been sent. It's a business and a security decision. You have already paid, had your card charged and entered all of the information for the IRS. To to have all that CANCELLED means you would have to be issued an automatic refund to your card, processes chosen like "pay with my refund" or "refund loans" would have to be undone; all of which would be rife for fraud, mistakes and security issues. So that's why it is just easier to make the policy, once you've transmitted that's it. If you need to change anything then you can do that through the IRS's Amend process only allows for one e-file.
Turbo Tax likes to give the example that it's like sending a package in the mail that you've already paid to be sent which I guess is an OK comparison. We can understand the post office not wanting to go through thousands of packages, pulling yours out and canceling the shipment. With an electronically filed tax return that's sitting in a server waiting for the IRS to accept it, there is no physical effort involved so why can't Turbo Tax simply pull the file off the server and give it back to you - again it's a business decision as to why that can't happen.
