Radio frequencies have no way of obeying geographical borders and licensing arrangements, and the extent of overspill depends on where broadcast transmitters are sited and their power.
In addition to traditional transmitters, overspill occurs when the footprint of a satellite is greater than that needed to serve its target audience.
[1] Transmitters located near to international borders may overspill into a large part of a neighbouring country, for example the signal from Republic of Ireland broadcaster 2RN's Clermont Carn site can be picked up in a large swathe of Northern Ireland, and vice versa BBC broadcasts can be picked up in the Republic.
Politically some governments may be wary of their own populace becoming too familiar with the culture of a neighbouring country or territory and feel threatened by it.
Cross border radio and television reception was an important influence on political developments in Germany during the Cold War.