Joe The DNR guy has a point. There is a distinction between taxing the general population, and charging a fee (call it a tax if you want to) that only hits the folks that use the service.
So trout stamp money is used to provide trout fishing opportunities. The trout stamp or fishing license isn't a tax since you choose to pay it in order to engage in the activity that the money provides.
They have a certain amount of money to spend to provide or enhance trout fishing opportunities. Planting lake trout in a lake that already gets stream trout might not be a good use of that money.