The American higher education system offers flexible options to students according to their needs and interests. At the undergraduate level, students can choose different courses before they declare their choice for major at the end of second year. This gives students time and options to explore their interests. So, if you are undecided about your field of study it is absolutely alright. You can still apply stating that you are undecided about your major. Also, if you have already decided what you want to study, you can still study another subject and complete a "double major", i.e. a degree in two subjects within the normal four year of study.
At the graduate level, i.e. master's degree, you can customize your course according to your coursework to fit your academic goals. You always have the freedom to include ideas and opinions important to your area of academic interest.