Easy in UK: invalid product description
I think you're 100% entitled to return the product for a FULL refund as the description does not make a mention of this and the product thus falls foul of the formal requirements for a product description (I think it's in the Trade Description Act 1968, but I could be wrong here).
In principle, the way WD advertises the product makes it appear there are no restrictions on usage and that's also a "reasonable expectation". Unless the restrictions are very clearly identified where the product is advertised (like on a website with an online retailer) the description of the product can be deemed misleading and are thus in breach.
The nicest way to make this clear to WD is if a number of these devices are sold and then returned, because the return cannot be at your cost as you were mislead, and the usual restocking fee should also not apply. If in doubt, consult your nearest Trading Standars office, which also has as nice side effect that they will act if they get multiple, identical claims.
Having said that, why bother? Just buy another make, it's not like there's a shortage of competition. In addition you may want to find an alternative that does NOT stand on its end like a book (a common 'designer' idea) - if this thing falls over when the disk is live (i.e. spinning) a headcrash is VERY likely, and that means bye bye to all the data on the disk. Bam - gone. It's a common 'feature' of most external drives as well.