BlogTO wrote a good post on our petition, below. It should be noted - that in response to Ryerson's claim of mercury leaks - that the sign was safely hung for decades without any incident.
Furthermore, if they deem that there is a risk, then the sign should indeed be fixed. But it's their responsibility to pay for repairs to make sure the sign is up to code. When you sign a contract and buy a house, for example, you are responsible for doing due diligence into how good the foundation is, etc. You can't come back to the former owner - 3 years later - and demand they fix your patio or something like that.
In this case, Ryerson agreed to put the sign back up. If they have problems with the safety of hanging it then they are responsible for repairs and bringing the sign up to code. It doesn't mean they can renege on the deal to put it back up.