The thing is that if Justices act too politically they jeopardize the sanctity of their own office. Congress has the ability to limit and restrict the type of cases that the Court can review. Also the court doesn't have any power to enforce the law... Which means that by playing really obscene political games they could seriously destabilize the country by making decisions people refuse to enforce. Finally the court can be increased in size. Get a gun enthusiast elected with a good Congressional majority and he can appoint as many judges as he wants. Which has the effect of making the Court a pretty useless body since it is just a matter of numbers.
All of these things put pressure on judges to moderate themselves. Not to mention the confirmation hearings.