Whether it's legal doesn't matter very much in practice if you lack the means to enforce your theoretical legal rights. For small sums, it's not worth the time to go to court for most people. In the US legal system, it's probably not worth it even for quite large sums, if you're likely to get stuck with paying significant legal bills even if you win.
For serious money, it probably becomes worthwhile to take action in apparently black and white cases like this, but I suspect many advertisers using Facebook ads (and Google etc.) are small outfits that never really spend serious money. They can effectively be mistreated with impunity regardless of anything the law says, unless they have recourse to something like charging back the payment for the campaign and relying on repossession of the funds being 9/10 of the law.
That assumes Facebook exists in your jurisdiction so there is a legal entity you can actually take to court, though. A significant difficulty with small claims processes, at least where I am, is that because you typically don't involve lawyers on either side so you can keep the costs down, you can also have difficulty even figuring out who you actually have an actionable complaint against if you're the little guy and you're up against a business with a non-trivial structure.
Whether it's legal doesn't matter very much in practice if you lack the means to enforce your theoretical legal rights. For small sums, it's not worth the time to go to court for most people. In the US legal system, it's probably not worth it even for quite large sums, if you're likely to get stuck with paying significant legal bills even if you win.
For serious money, it probably becomes worthwhile to take action in apparently black and white cases like this, but I suspect many advertisers using Facebook ads (and Google etc.) are small outfits that never really spend serious money. They can effectively be mistreated with impunity regardless of anything the law says, unless they have recourse to something like charging back the payment for the campaign and relying on repossession of the funds being 9/10 of the law.