What is the reasoning behind the resonating structure of halo-benzene?
I think the misunderstanding here (it's a common one) is over the interpretation of resonance structures.

What you have to understand about resonance structures is that they do not actually exist in the molecule. The separate resonance structures are never formed on their own but they can be used to represent a delocalised bonding situation, more accurately described using molecular orbitals. So electrons do not actually move from the halogen to the $\alpha$ and $\beta$ carbons but what the resonance structures represent is that, only considering mesomeric effects, there is a greater electron density on the ortho and para carbons and than on the meta carbons, the carbon attatched to the halogen and the halogen itself.



However, this is not the full picture because halogens; especially fluorine and chlorine, have a very strong inductive effect and so there is actually considerably more electron density on the halogen than on any other atom.

Electrostatic potential map for chlorobenzene: