You are right, People don't have to do anything. If you find something that helps you keep on track, be happyer, and improve your life. Why not. I am a Soka Gakkai Buddhist and I joined because it made sense to me, cause and effect, plus I support the organization goals for a more harmonious society. As in our Charter:
Purposes and Principles
1. SGI shall contribute to peace, culture and education for the happiness and welfare of all humanity based on the Buddhist respect for the sanctity of life.
2. SGI, based on the ideal of world citizenship, shall safeguard fundamental human rights and not discriminate against any individual on any grounds.
3. SGI shall respect and protect the freedom of religion and religious expression.
4. SGI shall promote an understanding of Nichiren Daishonin's Buddhism through grass-roots exchange, thereby contributing to individual happiness.
5. SGI shall, through its constituent organizations, encourage its members to contribute toward the prosperity of their respective societies as good citizens.
6. SGI shall respect the independence and autonomy of its constituent organizations in accordance with the conditions prevailing in each country.
7. SGI shall, based on the Buddhist spirit of tolerance, respect other religions, engage in dialogue and work together with them toward the resolution of fundamental issues concerning humanity.
8. SGI shall respect cultural diversity and promote cultural exchange, thereby creating an international society of mutual understanding and harmony.
9. SGI shall promote, based on the Buddhist ideal of symbiosis, the protection of nature and environment.
10. SGI shall contribute to the promotion of education, in pursuit of truth as well as the development of scholarship, to enable all people to cultivate their individual character and enjoy fulfilling and happy lives.