I guess that's because they are made so that you plug a connector on a drive, not plug the drive into the connector 
That doesn't make a difference - the connectors are designed to align themselves as they are inserted - backplane mounted SATA/SAS connectors are used on thousands, if not hundreds of thousands of servers and NAS/SAN storage devices, and have been used for over two decades.
If there is a problem on the ReadyNAS units, it's going to be in ReadyNAS fabrication or quality control.
For what it's worth, I also own a ReadyNAS unit (two bay) and have managed to break the release handles on both caddies - in the twelve years I've owned it, I've never had a drive failure, drives have only been removed for capacity upgrades and there have only been three such upgrades - 500GB to 1TB, 1TB to 2TB, 2TB to 8TB - so pretty much three removals. I've certainly swapped disks in/out of my IBM/Lenovo servers a lot more than that, and the same goes for my Equallogic iSCSI array.
Yes, the ReadyNAS is not in the same price range/market segment as the other equipment, so you know it's not the design of the connector, it's the quality of the construction.