raising after an all in... question

[Duck]

I'm not sure if i'm posting this in the right place, but could someone answer this question for me...

Why is it that in NLHE or NL Tourneys when someone goes all in and it does not constitute a "full legal" raise, you cannot go over the top.

For example: you bet $5, player B goes all in for $5.50 and player C calls but has $100 left. Why is it that when it comes back to you, you aren't able to raise it again?

I don't understand the logic behind the rule.... does it prevent some kind of cheating or am i just not seeing something plainly obvious?