Please note that nowhere in the scriptures does Jesus indicate that the salvation of souls and eternal life is reached by eating the bread and drinking the wine. He said to do it in remembrance of Him.
Someone who has not sworn fealty to the Catholic church can't take communion in remembrance of Jesus and be recognized as doing so by Christ?
Only in Luke.
Matthew 26:
26And while they were eating, Jesus having taken the bread, and having blessed, did brake, and was giving to the disciples, and said, `Take, eat, this is my body;'
27and having taken the cup, and having given thanks, he gave to them, saying, `Drink ye of it -- all;
28for this is my blood of the new covenant, that for many is being poured out -- to remission of sins;
29and I say to you, that I may not drink henceforth on this produce of the vine, till that day when I may drink it with you new in the reign of my Father.'
Mark 14:
22And as they are eating, Jesus having taken bread, having blessed, brake, and gave to them, and said, `Take, eat; this is my body.'
23And having taken the cup, having given thanks, he gave to them, and they drank of it -- all;
24and he said to them, `This is my blood of the new covenant, which for many is being poured out;
25verily I say to you, that no more may I drink of the produce of the vine till that day when I may drink it new in the reign of God.'
Moreover, in John 6, Christ is very explicit that we must eat His flesh and drink His blood, or there is no life in us. His listeners challenge Him repeatedly - He never says, "this is a parable" as He did in so many other places. And many were so revulsed - "this word is hard - who is able to hear it?" - that they turned away and walked with him no more.
As for your other question, to paraphrase C.S.Lewis, it's not that the Catholic church is right and everybody else is all wrong. The Catholic Church contains the fullness of faith as established by Christ (that's why it's called the universal church). But if one doesn't believe in the Catholic church, he shouldn't take communion there, because he would be eating and drinking his own damnation.
Similarly, there are other churches with closed communions, either because it's restricted to members in good standing (some Baptist, Mennonites, LDS, and JWs) or because of the theology of the Eucharist (conservative Lutheran and Greek or Russian Orthodox). I would never violate their church order were I to visit with them.