Because you don't have to vote for him during the presidential election. If you really believe that voting is immoral because you're participating in a system that breaks moral law by its very existence then you can participate in the nomination process, which doesn't assign actual positions of political power, without violating your own principles.
If I did not have to assent to "voter registration" providing a form of consensual evidence to State, Inc. in order to participate in this alleged nomination process, which doesn't assign actual positions of political power, without violating my own principals.... I might accept your well reasoned argument.
In the meantime I will stick to my Pom Pom's for Paul and let consenting registered citizens, or is it cattle, carry the voting water.