You are not obligated by law to buy lottery tickets, therefore, it cannot, by any stretch of the imagination, be called a tax. Period. They can advertise all they want. You can still say "no".
In fact, you can stand in front of a government lottery official, extend your arm out, point a finger at them and laugh out hysterically while loudly proclaiming that you will not buy their tickets. Try telling the IRS that you will not pay taxes because, well, you don't want to.