The lawsuits say taxing capital gains is unconstitutional because capital gains are property, and all property must be taxed at a uniform rate in Washington because of a 1933 state Supreme Court decision.
I haven’t yet found the context for this decision but I suspect this was put in place to ensure the single tax (ground rent/land value tax) was never given a chance. Mason Gaffney has written about how economics has treated the idea of land as a part of capital: a moment’s thought would disprove that, as land predates labor and capital, as the fruit of labor, comes even later than that.
Labor is prior to, and independent of, capital. Capital is only the fruit of labor, and could never have existed if labor had not first existed. Labor is the superior of capital, and deserves much the higher consideration. — Abraham Lincoln