Because in many respects, they aren't subject to the same laws as the rest of us.
Depending upon where, reservations are lawless lands. Federal law prohibits the tribal counsel from having or enforcing any laws about serious crimes such as rape, assault and murder, felony theft and so forth. Federal law also prohibits local, county and state law enforcement from enforcing any criminal laws. The interest of the Justice Department or FBI in crimes against Native Americans is next to zero.
As one result, on some reservations the percentage of women who are raped in her lifetime - and often repeatedly - approaches 100%. While most crimes are between Native Americans, this also is understood outside the reservation. If a man wants to rape anyone, go do it on a reservation. Unless he is caught by relatives, he got away with it. If a girl or woman reports being sexually assaulted or otherwise assaulted, she can mail a form in to the FBI or Justice Department. They will send back a letter stating they are investigating and give her a case number. 99% of the time that's the end of it as for justice. But there also then is a highly likelihood she will be retaliated against by who ever assaulted her - and possibly even more men and worse. If someone(s) who are not on the reservation did the rape(s)/assault(s), nothing will happen at all.
About the only thing the federal government has ever done in the past about rape of Native American women is to offer Native American teens and women $500 if she will be sterilized, thus preventing her becoming pregnant by rape. Other than that, for most reservations there is no other "law" about rape - nor can there be any by federal law.
However, this also makes some of the remote and impoverished reservations (not all) extremely dangerous for outsiders and particularly white people. Some have the attitude they will do to you what your people did to their people. Their actions will match their attitudes.
The type of unfairness you refer would be the same unfairness if there were no police and no courts for your city - and it also illegal for any of you to organize your own system of law and order to. Then it is accurate you would not likely be arrested unless it serious enough for the FBI to send agents to investigate and worth the trouble to take it to Federal court. It also is accurate that you could be assaulted and victimized by anyone who could get away with it. If you didn't like having been beaten, raped and robbed you could fill out a form and mail it in if that would make you feel better. But the assailant(s) might then get a phone call or letter about it, and they might be angry at you and come after you again and worse. Why not? Nothing stops them.
It isn't this way on reservations because that is how Native Americans want it. It is that way going way, way back to prevent Native Americans having any power and to make them easy victims. Later, the reasoning given was to prevent local authorities from abusing them. Either way, it is federally dictated lawlessness.