There is indeed the law, and at the end of the day that is what determines what you must pay.
The very wealthy have long supported income tax structures that treat capital gains very differently than employment income, with much higher rates being applied to employment income. For the most part, the very wealthy do not receive much employment income. This means that as a percentage of income (all types added together), people who primarily earn via capital gains pay far less tax than those who primarily earn via employment.
It is an understandable position to say that this tax structure is unfair and that the very wealthy are not paying what they should pay. You can disagree with it but it is not a difficult position to understand.