"I live on the east coast of the United States."
Translation:Mi loĝas ĉe la orienta marbordo de Usono.
«Mi loĝas ĉe la orienta marbordo de la Usono.» Is rejected, why does keeping the «the» is wrong?
sur describes something being on top of or in physical contact with (such as a coat on a hanger). ĉe describes something being the the immediate vicinity. The English phrase "on the coast" means "in the vicinity of the ocean", so it's ĉe.
this is true, but it's not inaccurate for someone to literally live ON the coast.
Your logic makes good sense, but that being said, "Sur" probably should be accepted. It's what the sentence says even if it's not what they intended the sentence to say. Languages are ambiguous.
How many people are living directly on the border between land and sea? Does the house get flooded at high tide?
The coast is not the literal border between land and sea. If it were then it would change with every wave and tide.
My concept of marbordo is exactly that -- the transition between land and sea --- not necessarily where the water is lapping up at any given moment, but the area of transition. Most people don't build houses there.
"ĉe" means something like "near". They live near the water - not in it.