Not just in startups. This is true for any transaction or business agreement (including plain vanilla employment) where your counterpart enjoys a massive information and power advantage over you.

I don't think any information or power disparity is required for this. If someone (person, or company, or government) can legally get money out of you, or pay you less, they will. Just like if you had an opportunity to leave your company and take $10,000 or $100,000 with you, it's hard to imagine two identical scenarios where you take the smaller amount.