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Glaives, guisarmes, lances, longspears, ranseurs, and whips are reach weapons. A reach weapon is a melee weapon that allows its wielder to strike at targets that aren't adjacent to him. Most reach weapons double the wielder's natural reach, meaning that a typical Small or Medium wielder of such a weapon can attack a creature 10 feet away, but not a creature in an adjacent square. A typical Large character wielding a reach weapon of the appropriate size can attack a creature 15 or 20 feet away, but not adjacent creatures or creatures up to 10 feet away.
If the Gargantuan T-Rex is adjacent to you, you're going to have to step back 5ft to stab it with your Longspear.
The whole reach thing is silly.
Never in the history of war has a human been unable to hit another human, 5' away, with a glaive or any other polearm. In fact, they can hit each other effectively at distances less than 5'. Likewise, no glaive has ever been long enough that a human could hit another human 10' effectively.
It might have made sense to apply a small penalty to a polearm user when attacking an adjacent foe, but absolutely prohibited is unrealistic enough to be wrong. Even moreso when you remember that two adjacent squares can mean those humans are moving around, each within his own 25 square feet of space (basically the size of a typical elevator) and there is nothing stopping the polearm guy from being at the back edge of his square, meaning his foe is at least 4 feet away, more than enough room to comfortably use a polearm.
/rant
Given the rules are written as they are, with some magical force making it impossible to strike an adjacent creature and with no provision for striking portions of large creatures that are farther away, you're stuck with having to create 5' of empty space between you and the creature to use your polearm.
Personally, I would make houserules to fix these problems, but those would just be houserules.
I think it is questionable. That section of the rules does say adjacent, which strictest reading would make it prohibited. Attacking a creature in a square is a strange phrasing though. It could mean if a creature is in any square adjacent to you, but it could also mean that the particular square you are attacking a creature in can't be adjacent to you, regardless of that other squares the creature is in.
One thing I will note, that if a large creature was invisible you target a square, and if that square contains a target, it is a valid attack (with all the miss chance etc.). It would seem odd for the rules to allow you to only attack a large creature next to you with a polearm if that creature was invisible.