Here's a more charitable way of thinking about this, based on my own experiences. When you're making a deal anyone (agreeing to do work, buy/sell, partner, etc.) there are two sets of motives you have to consider: what they tell you up front, and what they actually want. In your day to day, even at work, those two things are the same. But in many contexts where you would be talking to a "business person," those two things can be very different.
Lumping it all into "business person" isn't very useful because there are a lot of business people. So maybe a few specific examples can help.
Account managers and salespeople: while their stated job is to help you with customer issues, they're actually paid to sell/upsell you, and even if they're not, they're evaluated on selling/upselling you. This means they're likely to stretch the truth about the product they're selling in order to close the deal. So when you're interacting with salespeople, you can never 100% trust what they tell you, because they're not actually trying to help you.
Business development: the company they work for has an agenda. That agenda won't be obvious to you. Maybe they're asking you to come in to do a demo because they want to consider partnering with/acquiring your startup. Actually, they're spinning up a team to just build it themselves, but they want ideas. The problem is you have no way of knowing which is which.
HR: you know the common wisdom, they're there to protect the company, not you, and often that makes a huge difference.
Someone hiring you for contract work: they do want the work done, but they're trying their hardest to avoid paying. That means every ambiguity in the agreement becomes something they might try to leverage against you.
In other words, don't trust someone until you understand their hidden motive. "Business people" just tend to have more of them than your average person.