Seeming as ad agencies pay a great deal to reach my eyeballs, does that mean that I'm obligated to buy their products?

I suppose a big part of the divide is whether you see B&M shops as serving the business or the customer. If it is there to serve the customer, then their services should be paid for. If their services are there for the betterment of the business, then the customer should not be obligated to pay for those services.

I see business decisions as something that serve the business. If a business thinks that mailorder will reduce expenses and improve profits, then they should base their decision on that. If a business thinks that retail space will increase their sales and improve profits, then they should base their decision on that. Likewise for employees. If a business thinks that low cost sales staff who do little more than direct customers to an aisle will reduce costs and improve profitability, then they should go that route. If a business thinks that knowledgable sales representatives will increase sales and improve profitability, then they should base their decision on that. Notice that the customer's interests never (directly) enter the picture here.

If a business owner thinks that B&M is the way to go, then their job is to justify that position to their potential customers. Insisting that comparison shopping, erm "showrooming", is genteel shoplifting strikes me as being to the detriment of the customer and for the benefit of the business. I'm sorry, but I'm not obligated to pay for those business decisions if I walk out of a store and buy online because I discover a cheaper price elsewhere.