Hand of the night... #1

I just don't get the CO call on the river. Doesn't he realize that I'm going to showdown the hand regardless because of the MP1 all-in? I don't understand calling $7.50 on the river there with Ace high... Regardless, this hand made my night's profit (or most of it):

Full Tilt No-Limit Hold'em, $0.25 BB (9 handed) - Full-Tilt Converter Tool from http://www.flopturnriver.com

UTG+1 ($5.15)
MP1 ($5)
MP2 ($25.66)
Hero (MP3) ($47.32)
CO ($26.45)
Button ($25.23)
SB ($3.95)
BB ($27.15)
UTG ($5.65)

Preflop: Hero is MP3 with Ac, Qd
2 folds, MP1 bets $1, 1 fold, Hero calls $1, CO calls $1, 3 folds

Flop: ($3.35) Qs, 6s, 8d (3 players)
MP1 bets $4 (All-In), Hero calls $4, CO calls $4

Turn: ($15.35) 10d (3 players, 1 all-in)
Hero bets $5, CO calls $5

River: ($25.35) Qc (3 players, 1 all-in)
Hero bets $7.50, CO calls $7.50

Total pot: $40.35 | Rake: $2.01

Results:
MP1 had Ks, Ad (one pair, Queens).
Hero had Ac, Qd (three of a kind, Queens).
CO had As, Js (one pair, Queens).
Outcome: Hero won $38.34