If one percent of people call up customer service to complain about 50% of their transactions they are clogging up the system. I only call if something is breaking and I've exhausted all troubleshooting and automated methods.
It could be that fringe group of customers who use international credit card numbers (but also happen to spend well).
It could be elderly consumers who consistently get bitten by a rough edge in the UX.
It could be users who don't use English as their primary language and don't understand a colloquialism in the interface.
Or the elderly customers who call just because they want someone to talk to. I used to do some PCI compliance work and occasionally had to listen in on customer calls to call centers (to verify procedures were being followed). They don't want service. They want to chat. They may have some small issue with the UI, but then they want to add something to the order. Does it come in red? Will it match the one I ordered last week? Will the delivery people set it up? Can I make sure it doesn't arrive on monday because I'm going to the doctor?
(But I didn't listen to the phone calls, so what do I know?)
But the danger of - "I don't complain excessively so it's OK to whoever complains most" is that by gradually pealing off "excess" complainers, you'll get to a point where no can complain to any degree, people fear to complain about even wholly inadequate products for fear of getting on lists, etc.
Treating everyone with some base level of respect and fairness is the only way to ensure you, or anyone, isn’t subjected to awful outcomes from time to time.
Edit: and makes our call takers feel dumb with low resolution rates