If 90% of the population votes to elect a politician who will infringe the rights of the other 10%, there's not much that 10% can do (working within the system, at least) to change it.
History says otherwise.
In addition, laws limit how power can be used.
Because the Constitution, the Bill of Rights, States' rights, and laws about Congress officially declaring a war before fighting it are doing a wonderful job of limiting Federal power.
If 90% of the population chooses Acme Protection Agency, that has no effect whatsoever on the other 10% who choose to contract with someone else, or no one at all.
Sure it does. If those 10% have a "dispute" with the 90%, they will have to deal with Acme Protection Agency. What happens if they have discriminatory policies towards that 10%, especially those that don't have an agency behind them?
Yes, if 100 people who all happen to use the minority agency get in a dispute with 100 people who all happen to use the Acme Agency, chances are that Acme's policies etc. will be used primarily.
Also, if such a vast majority of the population were with one Agency, it must do business damn well. If the small one were much better, it would have more customers. Maybe the smaller agency specializes, say, in being a Protection Agency for people with handicaps.
That said, with Protection Agencies, you have a choice. The agency does not come to your house, impose its will upon you,and make you pay for its services "for your own good." That, I believe, is the heart of the matter.