According to "hardmac.com"...
...(they reported the original problem some years ago) this is the reason:
"Here is also an explanation of the reasons that pushed Apple to introduce that system in the 2009 iMac. As we sais earlier, the iMac checks very often the temperature of the hard drive. If it was doing so with the SMART system, it would shrink the disk bandwidth and would freeze the disk for a very short time at every check.
Therefore Apple decided to do it with an Out of Bandwidth system, outside of the data channels of the disks. At first the company used connectors added to most hard drives and used for programming and testing the disk. That solution had one drawback as each manufacturer had its own connector. The 2011 model uses a new system. All the information now goes through pin 11 of the power supply connector, which is normally used to light-up a LED during disk activity. In prodder to change the use of that pin, a specific firmware was needed, which explains the reason that only disks sold by Apple in the iMac don't have any problem."
...which is a reason, though I couldn't say whether it is a good one.