For the customer that's an option yes, but for the restaurant not so much. If you have a look at the fineprint of the blender it will say something in the line of "For private use only". This is not just because they want to extract more money from restaurants, it's because the engine is so poorly cooled and low quality components that it will melt if you run it longer than 10 minutes without letting it rest.

So yes, the customer can go home and make their own milkshake (unless they are on a trip and can't go home to make milkshake). But the store owner can't depend on a consumer grade blender to make 50 milkshakes in an hour.