Those batteries are radio receiver packs. Notice the tiny wire size...and the "NiMH 6.0V 4200mAh Sub-C Flat Receiver Battery Pack" in the description.

While they may be suitable to drive multiple servos in large aircraft (hence the 'high discharge' vs normal AA receiver packs), they probably aren't designed to discharge at the insane rate that the brushless motor/ESC combo require.

Not all sub C cells are the same.