If I find a product I want to buy in a store I will buy it there. More often than not, though, retailers don't have what I need and I end up forced to buy online anyway.
If I find a significant price difference, I'm going to buy it online. However, I will go out of my way to purchase at small business because there are benefits to that.
The fact that you bothered to go to the store when you could have just browsed their on-line catalog from your home is enough justification, IMO.
A lower price online is justified because there is less overhead.
Taking advantage of all those things and then robbing them of a sale isn't fair to the business trying to stay afloat. Comparison shopping between proprietors on equal footing is fair. Not between brick-and-mortar and a warehouse operation with huge economies of scale.
It makes perfect sense; there's definitely a tragedy of the commons involved, but its created by the store owner giving away their compelling differentiating offering rather than selling it.
OTOH, its hard to see what the viable alternative is.
How so? Why is it my responsibility as a consumer to ensure that certain companies stay in business?