They harm you by making this kind of behavior acceptable. You may not care about X (who might be spamming people to download an app even thought you've rejected

They harm you by making this kind of behavior acceptable. You may not care about X (who might be spamming people to download an app even thought you've rejected over a hundred times), but suddenly Y (that you love) sees how successful X has become doing this sort of behavior and begin to engage in the same behavior because there's little to no backlash in doing so. This is especially a problem when it comes to privacy because the average person doesn't understand the current and future dangers of the information being collected, and you're often forced to use these evil services because that's where everybody else is.