It is a bug; so that's the correct term. This however, has only been true for a short while.
When SoF landed, and mobs started killing us poor rangers from the tier 2+ through our WS, the message was that : "Strikethrough is intented to do just that; strikethrough - so live with it... or rather, not.."
That changed lately though, I believe it was at fan fair, where a dev suddenly changed and said, it was not supposed to strikethrough WS, and was considered a bug that would be fixed. Much rejoice.
Either way can be argued to be right - yes; it is a Strikethrough, so I guess it should be doing just that. However, the WS for rangers is not a tanking ability, rather a lifesaving ability for when the proverbial Sheisse hits the fan (poor guy - he's just a fan!). With mobs hitting 10k+ combined with the ranger's non-existing mitigation, this means as we usually hit this disc when already low on HP, a single strikethrough has a pretty decent chance at killing us off, rendering the disc worthless - more often that not have I died before even being able to let my group/raid know that I was WS'ing
.
So yeah; it's currently flagged as a bug - let's hope it gets fixed (and you guys' new toy as well; this is not meant for tanking for you either - rather a lifesaver for you/your group/your raid... Strikethough reduces that function by miles, though you guys have a much bigger shot at surviving a Strikethrough)