This is a classic case of fixing a tax small enough so that each individual family will not feel much pain, but will benefit the interested parties enough that they will lobby for it.
As Condy Raguet wrote in Essay No. XCVII of his Essays on the Principles of Free Trade, published in 1831,
Every one who has examined the subject, knows, that the reason why restrictive laws have been introduced into the commercial policy of most nations, is, that those who have a great and direct interest in the enactment can always bring their influence and power to bear upon the government more efficiently, than those, even thou vastly more numerous, whose interest is small and indirect (Itallics in the original).
Even if the per person cost of the tax is small, it is bad on principle. If live Christmas tree growers want to have someone study ways to increase demand for their product, they can fund it themselves. They have no right to have the USDA coercively take money from someone--even if it is only fifteen cents. No where is it written in Scripture, "Thou shalt not steal, except if it is a really small amount."