While looking over the Greyhound website, I thought I would check their weapons policy; however, I was only able to find this short quote:

On Board Restrictions
For everyone's safety and comfort, Greyhound asks that customers please follow certain restrictions while on board. Federal law does not permit smoking on Greyhound buses. We have a zero tolerance for alcohol, drugs, weapons and unruly behavior.
No other reference or legal reasoning is provided. Surprisingly Greyhound Canada has more information on their site: http://www.greyhound.ca/en/travel_in.../baggage.shtml