This is not taking advantage of free market capitalism, it is gaming the system. The free market allows for competition and provides a way for consumers and suppliers to come to mutually acceptable arrangements for a transfer of goods and/or services.
In general, smaller shops that provide fitting services will usually have higher prices than online retailers and big box stores that have the scale and market to support larger purchases and carry more inventory. You are buying two different things from a service focused vendor and a price focused vendor. With the service oriented shop, you are buying service, skill, and knowledge before and after the sale as well as the product itself, in this case boots. With the price focused store (ie. big box), you are buying the product itself only...a commodity. So, if you are ordering on line you are either stealing a service (getting something without paying for it) by sizing the boots at a different shop and then buying it on line, or you are counting on their return/exchange policy to achieve the same goal (guess at size, order try on, exchange and repeat if you got it wrong the first time). In the case of the latter you have to decide how much of a headache it is to you and whether the online price is worth it. If you have a particular brand that fits you with little to no adjustment and you can call the online retailer or the manufacturer to determine whether the fit of the new model you are considering is fundamentally the same as the previous model, then it really is a commodity purchase.
Why does this matter? It costs people jobs, and it costs consumers choice. By driving small service oriented shops out of business, or away from a particular business, it reduces supply for the product+service option and increases its cost.
I can think of two types of people who would do this:
1) Someone who can afford to pay for the product with the fitting but chooses to game the system because they are cheap and self centered.
2) Someone who can't afford to pay for the product with the fitting. Someone already outlined the way that this person can do right by the shop, and hopefully in the future they can afford to buy from the shop with the service they need.
As I mentioned, for those who do not need the service, they should just go to the cheap supplier because otherwise they are paying for something they don't need.
To be honest with you, I was being a bit tongue in cheek with that comment. That should have been obvious given the tone in my text.