I simply matched customer energy. If you were nice and polite I'm nice and polite. If you threw your money at me I'm hucking it across the store and you can eat shit.
Is it considered impolite to place it on the counter? I mean I get it if they had their hand out, but if they're just looking at me or busy with their hands fair fame.
Counterpoint, if the customer places the money on the counter for payment, how is there any issue if the cashier returns change by placing it on the counter?
What is considered polite may differ by region and culture. (I'm not claiming you specifically would be upset at this, it's just relevant to the meme.)