Well, it's not "free forever" - it's "free forever" as long as you have an active PS+ subscription. (Although if you let it lapse and come back - the "purchased via PS+" titles aren't removed they're just paused until you're PS+ again)
Your wording would imply you could sign up for 1-3 months, get everything for those months (the 1 month is only available as the trial cards to my knowledge FYI) - then cancel and keep it - which is important to be clear it's not.
It's an awesome ploy for them though, as you start building a critical mass of games it becomes harder and harder to ever want to cancel it - since month one you lose 6 games or so but when you're sitting at close to two years like I am (I started before the "upgrade" last year that made it awesome as hell) it's literally probably 100 games I'd be losing out on for less than a EQ1 subscription was. Like literally 2 hrs of play a month would justify it, and with so many games thats improbable to ever be a losing deal. [In fact so much so I'm sitting at a cancel date of somewhere late in 2014 for mine, I was already on a year + a trial card I added partway through when they had the $60/yr + $10 credit the next month + extra 3 months promo a bit ago - so I couldn't resist getting it squared for a long long time]