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Counting how many people come in to your store and how many of them actually but something is not "spying".

You also don't need client side spyware to track this information.

You don't need anything, but it's a useful way to do it without any significant drawbacks. In many cases, it's more or less the only option.

It is a significant drawback! It is literally not the only option to the situation you described.

Spying is the only solution for spying, yes. Spying is the only solution to over-eager people wanting mostly useless metrics.

It's not much different than me checking my Twitter "likes" constantly.