Perhaps sending and receiving are different per FCC rules; I'd like an example of how you mean that they are different. There are obvious things, such as sending is a much higher power situation than receiving; and the mixing frequency of a superhet design receiver might accidentally radiate and must be controlled while transmitting designs have no such other frequency to deal with.

But when it comes to any kind of simplex (one-way) communication, the receiving frequency must be the same as the transmitting frequency. Otherwise the receiver will not receive anything. Thus 433 MHz and 418 MHz units must be paired with units of the same frequency. An EU unit should not have the same transmitting frequency as a US unit, so must have a receiver to match.