I would argue that paid advertisement is a force distorting free speech. In a town square, if you can pay to have the loudest megaphone to speak over everyone else, soon everyone would either just shut up and leave or not be able to speak properly, leaving your voice the only voice in the conversation. Why should money be able to buy you that power?
why shouldn't it?
If somebody believes that their message is important enough to outbid everybody else, their message ought to be the one that is displayed.
The reality is that more often than not these messages are self serving and profit driven, many times borderline fraudulous in claims or questionable at best
the reality is that all messages, even those you think ought to be a grassroots message, are all self-serving. It's just self-serving for you as well as the message deliverer. And those "advertising" messages are self-serving, but not for you (or your tribe).
Therefore, this is just a thinly disguised way to try suppress the messages of those whose self-interest does not align with your own, rather than an altruistic reason.
Sometimes (often?) people with a lot of money may not believe in speech but in suppressing speech. However, money should not allow for suppressing speech, for example by buying a giant megaphone and speaking over people.
By your logic paying people $500 to heckle at your political opponents rally is fine. It may be legally okay, but it is a moral hazard, and for a better society we should try to better distinguish between “free” speech and “bought and paid for” speech.