Because free download products do not go through a processor and instead automatically authorize a valid download it would be possible for someone to copy the "buy" URL and share the link freely without your control.
As an example, PayPal products work like this:
1. Customer visits your page and clicks on a buy button
2. Buy button adds authentication information and passes the customer to PayPal to pay
3. Once Payment is complete, customer is sent back to DPD with their purchase information and DPD authorizes a download url that has time and download attempt limits to prevent sharing
Free products simply omit step #2, so when someone clicks on a free buy button they are automatically redirected to the special download URL in step 3 and the download is authorized.
Because of this, anyone could share the free "buy" url in step one and share the videos without your control. There is really no way to prevent this as any limits on free buy urls would require that you change the links to your videos on your site daily as the buy urls expire.
We do not currently know of any system that prevents "free" download link sharing.