It is a bit trickier than a hard and fast rule. It really comes down to one key thing, any injury a PTO player sustains during camp is on the Avs to foot the bill for. To fix the injury, pay for lost wages, pay for recovery, etc. Each team has insurance policies for that. It is very standard for these insurance policies to have guidelines like recovery time built into them for them to fall under the coverage (IE they don't want a guy a month after a surgery getting re-injured and having to foot the bill). NHL wide these are pretty cut and dry sorts of things, so in a way it is a rule... just not formalized. In this case, Maenalanen's surgery meant he would be disqualified from coverage. Which gets to a communication breakdown somewhere. Did his agent not inform the Avs? Did the Avs not inform him of the timeframe? Easily either could be the case and completely without malice is possible too.
Now teams can say screw it and not have the coverage if they choose. There is nothing stopping them from taking that risk. If we were talking Kane here, I'd bet the Avs take the risk. For a guy who is unlikely to make the team anyway and if he does is a plug... the juice isn't worth the squeeze.
This applies to Canadian teams too.