I think he was staggered not by the fact that this person was scavenging, but that they were scavenging during the boss fight.
Did this the other day on my sorc healer on the last boss of 55 MR. But I was healing and everyone was topped off (or close with puddle down), no one died, and I wasn't holding the group up any. I actually do a LOT of gathering when I heal FPs on my sorc, with scab and bio analysis, but if group gets ahead of me I just leave stuff behind. Hot+bubble tank, and sprint to catch up makes this relatively easy.
Then last night in a hammer station(tank and both DPS in guild), we had a merc healer who insisted on scavenging everything. We told him that's fine as long as it doesn't hold up the group, which it was because he was doing 0 healing. When he didn't cleanse the tank at all during first boss (resulting in tank dying twice), we kicked him and did the rest of the place with mako until a healer popped in right before the last boss.