Cait's traps inconsistencies

Well I was playing as Cait the other day and was up against a Morgana/Sivir, naturally I felt like any traps layed down would be free mana for the enemy Sivir so I didn't use it much, it caught my attention though, that when I placed one down Morgana used black shield, stepped on it and the damage/range increase was activated on morg, as a matter of fact she took heavy damage because she went after 2 and I managed to hit her twice.

This made me think that even if you spell shield the trap you'll get the damage off so it became trap town... well, I was wrong, for some reason when Sivir uses her spell shield it can completely swallow the trap without triggering cait's extra damage/range :S

so... uh.. wat?