The government is allowed to levy taxes as it sees fit, as long as the taxes are approved by the government (senate, House of Representatives, state governments, etc.) Some taxes also have to be approved by the citizens - there are budget issues that are put forward on the ballots at each election, each year. So, the government can pretty much tax whatever it wants, as long as the tax is approved.
Sin taxes were put into place years ago, and since they've remained a good source of income for the government, they keep getting increased. Sin taxes are taxes on items that are considered to be either bad for you (tobacco, alcohol), or luxuries (gasoline - you don't have to own a car or drive). Sin taxes mostly came about after Prohibition didn't work out, so they legalized alcohol again and then tried to make the taxes so high that it discouraged people from drinking - it became too expensive. That's sort of the logic behind sin taxes - to make things very expensive and therefore discourage their use.
Actually, the government could tax whatever it wanted to, as long as it got approved. But most people don't object to higher taxes on sin items, since they are on items that aren't good for you or are luxuries, and so all the non-smokers and non-drinkers, etc., will be in favor of them since it is a way to get more tax money that they won't have to pay.
We live a country that is ruled by a representative democracy. What that means is that we elect our representatives to look out for our best interests. We gave them the right to tax, make and enforce laws. If we lived in a true democracy every law every tax would have to be voted on by the citizens and passed by the majority. Our United States Constitution saw the folly in this type of government. That is why we have a representative democracy so the majority can't mistreat the minority. The minority is represented in our government.
Because there's no law that says that they are illegal.
At the Federal level, Article 1, Section 8 of the US Constitution grants Congress the power to lay virtually ANY tax. Alcohol, tobacco and fuel taxes are called excise taxes and Article 1, Section 8 specifically authorizes excises as well as any other undefined tax.
Nothing in the Constitution bars the states from laying any tax that the state chooses, other than taxes on other states. Most State Constitutions spell out what types of taxes that the state legislature may or may not impose. For example, the TX State Constitution bars income taxes but allows pretty much any other tax.