Bicycle Mechanics - Bottom Brackets

So i was just wondering something, i was talking to a person i met about bottom brackets, and he told me that on campagnolo cranks you have to have a campagnolo bottom bracket and the same goes for shimano,

why is this, shouldn't parts fit standard?

and the bottom bracket i have is a square taper shimano, i have an extra pretty good condition campagnolo crank that i want to put on the bike, but i haven't yet because of what he said

Campy square taper and every-other-brand square taper use slightly different angles to the taper.
Means that when you mix you don't really know where the crank will end up on the axle. It can be manageable, it can throw the chainline off, it can cause cranks to strike chainstays or bottom out against the BB cup before achieving sufficient torque.

As for "standard" parts, I think you're overinterpreting the value and meaning of the word...

Ignoring the positioning / clearence issues, the parts will fit together, no problem there. But it's not recommended to keep transferring cranks from one type of BB to another, because of this fit issue.

Campy square taper and every-other-brand square taper use slightly different angles to the taper.
Close but not quite correct. Campy (ISO) tapers and Shimano (JIS) tapers have the same 4° included angle but the ISO taper starts out with a slightly smaller dimension across the flats at the small end.

So, a Campy crank usually won't go adequately far up a JIS taper and a Shimano crank will usually go too far up an ISO taper. I say "usually" because the differences are small and tolerances sometimes let the cranks interchange adequately.

Try your Campy crank and see if it goes on the Shimano spindle far enough to be secure and if the chain line is good. It may work but there is no guarantee.

Here's a fairly common information source:
http://sheldonbrown.com/bbtaper.html
I think a BF member did some actual measuring and refuted some of Sheldon Browns claims. Is there a link to that somewheres?