It's not even about punishing United, it's about showing other players that if they behave in the same way, _they_ might randomly be subjected to this sort of punishment. People are not going to stop using airlines, but we might stop using yours for just long enough to make it hurt.
Even a vocal minority can have a profound impact on things. Look at how people got mobilized to let their representatives know they didn't like the ACA repeal bill which did end up having an effect on congressional members. Or more recently, how groups mobilized and pushed an ad boycott of Bill O'Reilly. There's even companies who opted to drop Ivanka Trump's line of fashion because of public outcry.
Same sentiment in this op-ed piece on Medium. https://medium.com/@thisTenqyuLife/united-airlines-should-go...