Why would an external drive specs list USB 3.0 for PC but USB 2.0 for Apple?

[glitch44]

I was looking at this Western Digital drive:

Western Digital® My Book® Essential™ USB 3.0 External Hard Drive, 3TB

But in the specs it says:

Compatible with Windows PCs. Also works with Mac OS X 10.5+, but requires reformatting and is limited to USB 2.0 speeds.

Huh? Why would the OS limit the drive speed?

Here's the product link:
http://www.officedepot.com/a/produc..._mmc=Affiliates-_-aB8BeTfRgb8-_-10-_-127125_1

 

[SDub90]

I can only find that specification on the office depot website - maybe the product was added before Macs had USB 3.0 and they added it so users didn't get confused when their Macs weren't getting USB 3.0 speeds?