There’s a big difference between buying ads to explain your political positions and buying elections. To help you understand this, let me explain how elections are actually bought. It's by literally bribing voters to vote for you.
I'll start with this historical example. Originally, Senators were chosen by state legislators. In 1899, William A. Clark, a millionaire "Copper King," bribed the Montana state legislature to elect him as US Senator. That's buying an election. It's also a big part of why voters, rather than legislatures, select Senators.
The traditional Chicago election includes a lot of "walking around money," which is used to bribe voters directly or to bribe people who have a lot of influence on voters. The classical direct vote buying method was chain voting. The man with the money hands the voter a marked ballot. The voter goes into the polling place, gets an unmarked ballot and puts the marked ballot in the voting box. Outside the polling place the voter hands the man with the money an unmarked ballot and gets paid.
What Sheldon Adelson does is the same thing George Soros, Tom Steyer or the AFLCIO does. They buy political ads, pay for political pamphlets and pay staffers to make phone calls and walk door to door to get out the vote. All of this is free speech, whether you agree with the message or not. If you don't like it, back the Democrat's attempt to amend the Bill of Rights.