Startups are quick to hire and fire because hiring exactly the right person is important, and hiring the wrong person can cripple the company. Everyone who applies for a position at a startup knows how this works, and I'd argue that if you aren't okay with this atmosphere, you need to avoid the startup scene and go work for a big company with some decent HR policies.
The bullshit shows up when a startup succeeds, and the company tries to preserve its startup culture because they want to keep growing like a startup. You then get BigCo politics with startup arbitrariness, and that's a recipe for disaster. Similarly, sometimes the Good Idea Fairy whispers in the ear of a high-up executive, and he tries to institute startup attitudes toward hiring and firing at a BigCo. This also goes poorly.