As a part of NATO, the United States has an obligation to defend its allies, regardless of how much or how little an individual ally contributes to the alliance. The entire purpose of an alliance is to have a collective of like-minded nations to support each other in times of need or crisis. The United States is a huge superpower with a great deal of influence and resources to contribute to other nations, but not all nations have the same level of resources at their disposal. This is simply a reality of entering into an agreement with other nations on an international level; not all nations will be able to contribute equally.
The United States made a promise to NATO that it would support all of the member countries. It is important for the US to be honorable and follow the promises that they have made. Otherwise there is no reason for any other country to follow the promises they have made to the US.
Yes, the NATO allies have a responsibility to each other. Not everyone can contribute in the same way, but it is important that these allies stick together and help each other out as they are able. The U.S. also needs to play its part and stick together with its allies.
The United States should defend all allies, regardless of their contributions financially. The whole point of the organization is security, stability and prosperity for all. Also, defending smaller states on the fringe of the alliance is the only real way to stop Russian aggression from expanding further than they already have.
If a country is not contributing to the alliance then, America should have no obligation to defend it. These countries are just making a profit off of the bigger countries. It could be a needless waste of life and precious resources that can not be wasted willy nilly on a country that would most likely not help them back if they where to get in trouble.