You can't blame Square-Enix for Infinite Undiscovery being Xbox 360 exclusive. The IP is owned by Microsoft. Originally the game was to be published by Microsoft Game Studios and Square-Enix had no part whatsoever at the beginning. Tri-Ace was developing it, Microsoft was the publisher and IP owner. But then Microsoft agreed to let Square-Enix handle publishing because they felt that Square-Enix had more expertise when it came to publishing jrpgs. MS came out of the deal retaining the IP rights so S-E couldn't publish it on another console.
The Last Remnant was also originally meant to be multi-plat. The PS3 version was scrapped because Square-Enix is too incompetent to figure out how to get Unreal Engine 3.0 to work on PS3 (the Japanese don't usually use middleware like that so they aren't used to it)
As for Star Ocean 4, Square-Enix said that Tri-Ace began development back when only the 360 SDK was available. PS3 SDK wasn't out then (so i take it this was 2005 or so). There's nothing unusual about that. The Xbox 360 received a number of exclusives and console exclusives early on because game devs received a 360 SDK before they could get their hands on a PS3 SDK (ie. Call of Duty 2, DDR, etc). Heck remember when FF13 was being developed on PS2? Game devs like Tri-Ace and Square's FF team gotta be kept busy, they have mouthes to feed at home. And so they work and and they use what's available at the time. And if you have MS sending over a SDK that allows for cheap and easy development for their console, why not give it a go? Especially if you are the size of Square-Enix and can afford to experiment. Square-Enix probably thought, "hey, why not? Let's start the project on 360 now, release it on PS3 later with extras to placate the base we have on the Playstation."