Bots provide the liquidity.

If they ban bots they will lose liquidity and then lose customers.

They already provide lots of warnings for using the free api without actually placing bets adn these may increase.

Before they ban bots they will remove the free api and make people pay to use the api. If they do this people will just convert to placing the bets using bots via screen scraping the front end. But this will actually be worse for server loads.