It is not allowed because if a neutral wire became disconnected, the return path for electric current could be along a ground wire. While that itself may not be a hazard, if that ground wire also became disconnected somewhere, parts of the ground system could be energized. That's not EVER supposed to happen.
The neutral wire is a "low-risk" return path for the electric current in that branch of the system. All of the neutral wires all have the same electrical potential... nothing. At least, no potential compared to ground. There is, of course, 120 volts of potential difference between a neutral wire and any hot wire in the residential system.
If you touched the metal part of a live neutral wire you should not receive a shock. (But don't try it!) By tying the neutral to ground at one point, half of the conductors (in a typical 120 volt circuit) have no dangerous electrical potential. Of course, the hot wires are still dangerous.
If you need further help or clarification on this answer, please do not hesitate to reply to this post.
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Best of Luck, Brian
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