What do you not understand about "It's private property and they aren't cops" ?
There is nothing illegal about me asking to search your backpack if you want to come into my house, and there is nothing illegal about me searching it if you give me consent to search it. If you say no, I'll just refuse to let you enter.
There is nothing illegal about the Air Canada Centre searching my backpack for weapons before I enter the stadium to watch a hockey game as long as I give consent for them to search it. If I say no, they'll just refuse to let me enter.
Same situation with condo parking lot security guards, there is nothing illegal about them searching the back of your windowless van as long as you give consent for them to search it. If you say no, then they'll just refuse to let you enter the parking lot.
It is a completely different situation when a cop wants to search you for walking down the street or your car at a traffic stop.