It's not the only way, but it depends on the way you're doing your USB drives. If you buy pre-built USB drives you could well be right -- I wound't know because I've never owned a pre-built external drive -- I always buy external enclosures and put my own drives into them. This way, I always know what quality of drives I'm using.
Also, some drives will spin down on their own, without being told to. This 'feature' is one reason people have trouble with, for example, WD Green drives. People connect Greenies to a RAID card and have trouble because the drives are spinning down and the RAID card thinks they've died because they don't respond fast enough, so it drops them out of the array.
Also, I believe some external enclosures, particularly multi-drive units, will spin drives down on their own. I will have to check this on my Lian-Li EX-503 enclosures although, to be honest, I'm really not that concerned if the drives run 24/7.