I'm very sure they can sell below the MSRP, they might not be allowed to advertise that lower price.
You are correct. They sell below MSRP through Greentoe and Canon Price Watch and probably though other means as well. I was typing that response on a Smartphone so I skipped the full explanation that I assumed most of the members of this forum would be aware of.
Adorama and B&H have both lowered prices below MSRP on occasions in the past by adding additional items to the main item. For one, last year I purchased a Canon 100-400 mk II for $350.00 under MSRP. The store had a package deal with the lens and a Canon Pro 100 printer. The printer was free and had a $350.00 rebate. So I got the lens for $350.00 off and also got a good quality printer that I haven't even opened the box to it yet. . .