Not only this, but while it's cool achievements are going to be account-wide and, hell, so are mounts, there are people who have worked toward it intermittently over the years to get to this achievement. Those people didn't ask for account-wide achievements. And now they're about to be screwed out of 310. I have one toon with all but two of the metas, one toon with 4 of the metas and another with 6. None of them have Noblegarden.
Is 4k per toon a lot? Well I have 10 85s, but really, I play maybe 4 and only 2 with any severity. Of course I can afford it. I just think it's a bit crappy.
Give me the choice between all achieves and mounts going account-wide and this specific one NOT including Master Riding account-wide and I frankly don't know which I prefer. Not because of the money, but because I finally established a main and stuck with it. Long, Strange Trip is one of those nagging things for me, the trip I take annually isn't something I'm actually doing next year, I was going to get it finally, without account-wide.
If they didn't remove the auto 310% riding then the only option would be to keep the achievement character specific.
So your choices would be -
A) Buy the skill like everyone else
or
B) Complete the hefty achievement on each toon that wanted 310% for free.
5k is nothing these days. Even less in MoP, it's hardly worth crying about.