I understand the purpose of a "subscription" based system, but it seems wrong that we have to pay (to renew subscriptions) in order to download a version that is just fixing a problem in your component.

You even state that the component is not working correctly:

3.6.7 - Resolve a permissions check in Joomla versions between 3.0.0 and 3.5.0, which caused the SIGPro editor plugin to break page rendering when editing content in a site's frontend.

Hmm..we are paying you to work on fixes that aren't supported?
Stop doing the above and add new features, like searching for site galleries in the admin area.

I have version 3.6.6 - so this isn't really an update for this version at all?

Hmm... interesting response for someone selling something.

I like SIGPro. Not wild about your subscription based system and having to pay to get a fix for your coding errors.
Charging for security fixes is my biggest complaint.

"This fix applies to old 3.x releases, which technically, should not be supported." Why did you do this?