What are you talking about? Countries on friendly terms *don’t* do that, unless the aircraft/vessel is unexpected and unidentified. Do you have some example to the contrary? Hostile or adversarial countries definitely do that for obvious reasons. If a plane or ship from a hostile country enters your airspace or waters, they could be spying or preparing to attack, and you don’t want that, so you send a plane/ship of your own to say “get out or we shoot”.
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