I'd be willing to bet that a higher percentage of LFR-only players toggle their subs on and off than do normal+ raiders.
So why should blizzard burn two-months of subs from those players?
"Oh, but I'll just quit and resub when it's open rawr rawr rawr"
"and all my friends do the same rawr rawr rawr"
"and I don't know what anecdotal evidence is rawr rawr rawr"
Great! But again, I'd be willing to bet blizzard has a bunch of data that correlates a sub drop off within a modest time of LFRs full release.
Blizzard is a company. They are out to make money. They aren't evil for doing that.
If they have designed a timeline which keeps you subbed and logging in to play a game, then they are providing you with entertainment and deserve your money.
If you don't like it, stop subbing. They are kind enough to let you toggle it on and off a month at a time. That's really pretty generous considering it means you could just wait out an entire xpac until the last month or two: the game itself only costs ~40% of the original price and you render 12+ months of subbing unnecessary by doing catch up mechanics for two hours.