Your point is correct, but your numbers are off.
150 seats requires 3 FAs, while 151 requires 4 FAs.
This subject was discussed extensively when B6 removed the one row of seats - the very high load factors at B6 apparently justify the extra 6 seats. Same at TED, which configured its A320s with 156 seats - plenty of opinion that UAL's FAs, who make much more money than the FAs at B6, would not make the extra 6 TED seats worth it.
B6 was barely profitable in 4Q2004 (and would have posted a loss if not for an accounting change) and there are rumors that B6 may post a loss in the 3rd quarter ending in two weeks. If so, perhaps B6 miscalculated on those 6 seats. Perhaps not.