I actually think that the most trustworthy casinos don't offer these sign-up bonuses.
It's quite simple actually - the sign-up bonuses are obviously tools for the casino to make money through marketing it as a really great thing but refuse to pay out anything once you actually win something (in most cases).
I don't think offering sign-up or welcome bonuses has anything to do with the trustworthiness of the casino! Many trustworthy casinos that I know of offer this kind of bonuses. In fact many businesses do it, not only casinos.
As you said, it's just a way to attract new customers.
It's not that they refuse to pay when you make a good profit, they just impose some requirements (wagering × times, deposit, kyc...) to ensure they will stay in profit and to prevent abuse.