HOLD YOUR HORSES EVERYBODY!
This is likely not a proprietary connector at all, but a new standard we're not familiar with.
Apple doesn't make hard drives. Therefore, someone must be making hard drives to fit in these machines. Apple doesn't want to rely on a single source, so that means multiple businesses are making these drives.
These people can sell these drives in the aftermarket to mac users.
Further, they aren't going to tool up and design a custom hard drive for Apple.
Apple has learned this lesson already. Apple does not forget lessons- especially painful ones!
This is going to turn out to be a standardized connector and part of the SATA Standard. It might not be the common, popular, backwards with SATA-1 connector... but nobody up and down the supply chain likes one off, specific designs.
The forces for standardization are VERY powerful.
And having read Apple news for the past 3 decades, I know a lot of BS gets out there, and people jump to conclusions only to later be proven wrong, but when it happens, everyone forgets that they were all up in arms about it (But the general "apple wants to screw you over" mantra persists.)
I've never been screwed over by Apple and I've never had to pay too much for any Apple part, device, accessory or product.
Apple is not going to make a one off hardware standard in order to get an extra $30 from the few people who open their iMacs to upgrade the drives.
Let's get real.