The "official" stand that I gather from Catholic websites is that non-Catholics taking communion in a Catholic Church is frowned upon because these people are not confirmed Catholics and don't follow prescribed rituals associated with the Mass. Many of the rank and file Catholics are not bothered and don't follow the prescribed rituals and leave participating in the RC Mass to individuals. Some Catholics who follow the ritual will sit out a Mass if they feel that they have not followed the prescribed rituals.
