You have to be careful giving Mr. Iron negatives, they can stack to no end. It's totally possible to get a Mr. Iron that gets negative experience, loses more than 100% xp to the forge, or costs you gold with each creep it kills.
I can see it gaining 0 exp, but gaining negative seems to be a bit much. This deleveled it to level 0 making it completely useless. Even the negative damage has a limit of -100% why not this too?
I agree, towers should not be reduced to level 0 or have an experience amount of less than 0. I filed a ticket to fix this for version 0.9.3 - Prevent towers from gaining negative experience (0000298)