Here comes yet another imputation! Knock.it.off. There is nothing unacceptable if many Christians receive communion in their denomination in memory of Christ.
Catholics believe transubstantiation occurs during the Mass and that the actual body, blood, divinity of Christ is being--not only received, but proclaimed. As Orthodox and Coptics also believe this, there is no reason for them not to receive the Eucharist, and they do receive communion in the Catholic Church.
However, if someone does not believe in Transubstantiation has occurred, it is not in anyone's interest to proclaim, in fact that did occur. No matter what denomination one embraces, I doubt anyone is up to lying when in another church.