Casinos cannot make any money on the welcome bonuses that they offer to the users. This is because welcome bonuses are a form of investment for the casino, and a means to bring in more users to the site. The only profit that they can make from the casino sign up bonus is long-term. If a player likes the terms offered by the bonus, signs up at the site, and becomes a loyal user of the site, only then would the profits come in.
Players might decide that they do not like the casino games, and leave the site once their bonus is completely used up. In this situation, the casino will not make any profit.