Did no one see this coming? Many other games have done this in the past; most famously WoW which has made the earlier expansions free.
The game is at the point where you NEED the expansion to enjoy the game and its mostly defunct content now. Most people only go to Makeb because they have to; dailies for money, still need 72's from TC etc.... S&V HM is about to be released and most of the other 55 stuff introduced is redundant or rehashed level 50 stuff like *yawn* TFB: \
In addition, in fact imho even though it could well be seen a cheeky to do it only 6months and not 12months or w/e; this is a good thing for those of us at endgame as it increases the pool of players. More 55's means more people to lolGroupFinder with or a bigger pool of players to recruit from. Any MMO is only as strong as the community that plays it and this only increase and enhances it.
Chances are if you did not have the expansion back then and where playing at endgame you would be bored stiff, barely anyone who is unhappy at this would not have got the expansion even if you knew it was going to be free. You would have paid or left rather than sit around bored for 6months wasting sub money not being to enjoy anything in game with it all being moved to 55.
Burning Crusade came out in 2007 and was given for free in 2011. Wrath of the Lich King came out in 2008 and was given for free in 2012. Neither of these expansions at any point were stated by the developers that they were going to be put out there for free. Initially the planet Makeb was. Not to mention there is a world of difference between 3 years and 6 months. Using WOW as an example doesnt really work here since they were 2 expansions ahead by the time either of those were released for free or they were released within a couple of weeks of those free releases. I somehow doubt that SWTOR is releasing a new expansion with increased level cap,gear and planets within the next month,much less two of them.