For example, they could start asking brands not to allow online sales if they want to be stocked; I can easily see the more expensive brands only being available off-line. Or they could ask people who enter the store to pay for a redeemable voucher.
And, as a consumer, I sure as hell am not going to pay a brick-and-mortar retailer for the "privilege" to shop in their store.
So, it can be a matter where maximizing your revenues now can hurt you in the long run. But, most importantly, as brick and mortar outlets fight for their future survival, somebody could choose to let go of his online sales to win a decisive hand in the traditional channel.
As a manufacturer, I don't see how shutting down my online presence would benefit me. Simplified, there are three types of customers. The first shop in traditional stores, find what they like, and purchase it. The second browse and purchase online, without ever seeing the physical merchandise. The third browse traditional "showrooms," then proceed to buy from a cheaper online retailer.
With an online and traditional presence, manufacturers capture all three types of customers. If you limit yourself to traditional stores, you force the third type of customer to purchase in-store, because there is no cheaper online alternative. But those customers would buy online anyway, so you gain nothing. And you've completely lost the second type of customer, who only shops online. A simplified model, perhaps, but the principle stands.
I'm honestly not too worried. I like going to the mall. My mother practically lives at Target and Wal-Mart. Many of us like the physical act of shopping, enough to sustain physical retail outlets. Profits may shrink, but that's the free market. You've got to keep innovating to stay in business.
The suppliers aren't stupid, they see where this is going. Any wholesaler who agrees to that clause deserves what happens next – being overtaken by their more forward-thinking competitors.