Because lots of politicians are fools

Greedy fools, at that.

Short version: "Let's pass this tax on cigarettes to make people stop smoking!  And we can use the money from the tax to pay for lots of things!  What?  Unconstitutional?  Aw, it's for people's own good, so sure it'll pass muster!"
It doesn't.
"What?  We were counting on that money!"

Let's start with "Let's use a tax to control behavior.  For their own good." 
Then throw in "What happens if it actually works?  All that tax money will go away, and then what?"  Which isn't answered.
As warned, the tax didn't pass muster.  Now they're whining "This leaves a hole in the budget, we were planning on that money!"

Morons.