Wal-Mart Fires Security Guard for Catching Thief

Since we are talking about dismissal for cause in my employment law course this week, I’d thought I’d note an American incident involving that bastion of progressive employment practices, Wal-Mart.
Security guard Josh Rutner saw a man walk out of the store with a box of golf balls and told him to stop, but the thief ran away.  Rutner and another guard then chased  down the thief in the parking lot and were surprised when the thief brandished a knife and swiped at them.  The two guards were nevertheless able to take down the man and hold him until police arrived.  The thief was arrested.  The next day, Rutner was fired by Wal-Mart for breaching a ‘no chase’ policy.  And if being dismissed for catching a thief is not bad enough, Wal-Mart went further and told Rutner he could never be hired by Wal-Mart again because his offense was unforgivable.
This is an American case, where generally employers can dismiss employees at any time, with no notice and with no reason under the ‘at will’ model of employment.  Could a Canadian Wal-Mart store dismiss a security guard for ’cause’ (without notice) for chasing a thief into the parking lot in violation of a company policy?  Maybe, if the policy was a term of his employment contract that the employee was made aware of, and the company consistently enforced the policy as a ‘zero tolerance’ offense.  In Canada, the question would be whether the employee’s breach of the contract ws sufficiently serious to warrant the employee forfeiting their entitlement to notice of the termination–in other words, did the employer have ’cause’ to summarily dismiss the employee?
Sometimes, Canadian judges have refused to find cause when an employee breaches a company rule, particularly if the rule was not made clear to the employee and there is some ambiguity in how it has been enforced.  In addition, if the employee can put forward a ‘reasonable excuse’ for the conduct, judges have sometimes sided with the employee, found no cause, and ordered reasonable notice damages be paid.
In the Wal-Mart case, the security guard said (to reporters and the employer, I assume), that everything happened fast and once the guy pulled a knife, he felt that he posed a risk to the public that he should try to address.  Do you think that should be considered a reasonable excuse for breaching a no chase rule?
Nice to know, by the way, that if I steal from Wal-Mart, that no one will be chasing me …