external drives for mac
the formatting of the drive does not really matter unless you want to use it with both Windows and Mac, in which case it will need to be formatted as FAT32 to work with both since Windows can't recognize Mac-formatted drives.
Now, in the event that you're looking for an external drive that you will want to be able to boot from in the event of an internal drive going bad, etc. then it is VERY important as to which you get.
Macs will not boot from USB / USB 2.0 external drives at all and will only boot from certain firewire drives. The firewire drive has to use the Oxford 9xx chipset for the Mac to be able to boot from them, so this could be the reason for the Mac-specific listing of drives on NewEgg.com.
If it's just going to be used for backup purpose only, then you can buy any drive you want and format it on a Windows machine as FAT32 if you indeed want to use it with both platforms, otherwise, it doesn't matter.