Why is Serial.print working when nobody receives the data?
The USB-UART bridge is, if it can. And if it can't, no one cares. A UART is asynchronous, hence it doesn't care if anyone consumes it; as long as it has sent the data, that's all it cares about. There can be additional checks to see if the other side is able to receive data, but that is not implemented on the Uno.