Theres many reasons people get mad so I'll go through them.

1. When a company sells a game as a console specific title made for the specific hardware. Sometimes one consoles specs are considered more desirable than another.

2. Even some third parties that were at one point exclusive stood out with a certain brand and represent them as well as any first party ever could.

3. Gamer specific consoles (public perception). (360= shooters, Racing /PS3= Fighting, Action, custom creation etc.)

In this case in reality a lot of Sony fanboys get pissed off when PS3 exclusives went to the 360. Why? Because they represented Sony amongst the first and second party exclusives. If this were the case vice versa with Microsoft, there would be very little that would have a true need to go multiplat outside of Mass Effect (which happened) and Gears.

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Ms doesnt help pc because there is no competition. Its not the otger way round. Pc devs are then more on the line. Sure maybe this results in more exertion to making a selling game, but you must also realize the sheer number of crap pc games. So much so that the only thing that really saved the pc market, was steam.
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Thats the thing. It is not proven thar the move to multiplat IN EVERY CASE will sell substantially enough to cover tge associated costs. Like i said exclusivity tends to add 10-20% sales on that one platform, for a variety of reasons including but not limited to, multiple console owners choosing which system to buy the title for. Look at ff13. Ps3 5.8m 360 sells 2m. 20% would result in an extra 1.16 m sold on ps3. So the question is, was ff13 profitable on 900k games sold on 360 alone.
the case is even worse with ff13-2. This is further compounded by se admitting that they cut out portions of the game to fit the dvd size constraint. The game might not have bwen absolutely wrangled for its linearity and might have sold more as a result so maybe potentially 30% more.
this is the inherent problem with many arguments about exclusivity. Exclusivvity is never called "the best solution" by proonents. It is a good solution depending on the circumstance. Obviously a game like cod sells best as multiplat, but others sell and perform much better as single plat. Adding to my previous arguments in many but not all circumstances exclusivity coupled with multiple consoles is the way to go.

Whether the PS3 version sold more isn't important. What's important is that the 360 version sold a considerable amount for a franchise that was for the longest time largely synonymous with the PS brand.

To me, it makes a difference in how the game is perceived and received. That's just my opinion.

Happy when the game was badhed for lacking content and square says the game was cut due to space limitations on dvd, you have to wonder how good the game could have sold as an exclusive. Especially considering that the lacking content is the games only flaw. It is a brilliant game but its obvious they cutout a lot.
Also factor that some might have bought the ps3 for the game and that at least some nultile console owners might have simply bought it for 360, you begin to wonder exactly how much of a boost would it have had had ot been exclusive.There is at least one guy that comitted sudoku because of the move and refused to buy the game on that merit alone.
and of i know the internet and fanboys there was likely a good deal of people who did the same.

He committed sudoku, are you serious?? Man that guy took it way too much to heart.

None of that ever occured. There were no cutbacks made to the PS3 version because of the 360 port. This has been debunked many time.

There was no cut content. That was how Square-Enix chose to design the game. They stated that many times and it has been relefected in everything they have released in recent years including their PSP games where everything is primarily focused on the narrative just like the 3rd birthday, Crisis Core and FInal Fantasy Zero.

No aspect of it being ported would cause  that type of content to be cut. No aspect of the game being on a DVD instead of bluray would cause that type of content do be cut.

PS where psychologically where a lot of people think of these titles loosely. The reason the 360 version sells considerably is because Microsoft used the 3rd parties and price to get the last gen PS2 gamers to make the jump from the PS3 to the 360. Too bad they cannot sustain things on the exclusives end.

Hes right that a lot of people expected the game exclusively on the PS3. When I solely owned a 360 at the beginning of this gen I felt like I could've been missing out. All the energy that was focused on making an exclusive version of Final Fantasy was turned over to Versus XIII which is why the PS3 is still getting hype for it. The weight is huge on this Versus title and it is still expected on the PS3. 

THIS

http://m.joystiq.com/2010/01/13/square-enix-cut-a-games-worth-of-content-from-final-fantasy-xii/
http://gamer.blorge.com/2012/01/07/square-enix-addresses-all-concerns-in-final-fantasy-xiii-2/