This is actually a serious issue I have with how telepathy is typically portrayed: nobody has any sort of mental defenses unless they are also a telepath. (I think source books for making the X-men into RPG characters essentially force you to take a small amount of telepathy when making such a character, just to represent that they actually have a nonzero save versus telepathy.)
My general feeling is that everybody (with small exceptions) should have a default small amount of telepathic defenses, just like how anybody who's not bedridden/asleep has at least a token amount of resistance to a physical attack.