Even then why is such culture legal? Over here you're legally required to let someone know they're under performing and give them a minimal time (I think 6 months in the UK?) to improve before you can fire them.
In general, Americans see this protection as none of the government's business. If the business wants to provide it, or unions want to demand it, that's fine, but it's not going to be applied as a blanket policy.
Many businesses do apply some sort of lengthy process for firing people, for the simple reason that experienced talent is valuable and removing it capriciously is a terrible idea, but it's not required.
In my experience, however, even these processes are just drawn-out kabuki dances. By the time that you've been formally notified that your performance is deficient, the decision has already been made to fire you and nothing will change their minds.
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As for whether this attitude is a good idea, I'm mixed. On the one hand, it allows businesses to take a risk and hire inexperienced or otherwise dubious prospects without fear that they'll be stuck paying a useless body for months after it's been found that they're a bad fit. On the other hand, it allows shitty managers to be thundering assholes and capriciously fuck with people's livelihoods.
However, I'd assert that the American attitude toward employment is critical to startups. A BigCo can be indifferent to keeping a shitty employee around for months. A startup has much, much less margin for error.