It just makes sense.
As the number of significant digits goes up, people's understanding of the extra meaning goes down. The first digit is the most important, the second less, and so on. Adding many significant digits is only used to distract people.
I don't think giving people the economic freedom to exploit gullability of the population is beneficial. I think that's a popular idea, too - there are quite a few other laws of the same type. For example, laws against cigarette advertising, bait and switch sales. It's also the reason we make things like heroin illegal. In all those cases if people behaved reasonably and responsibly and made sure of things themselves, there wouldn't be a problem. Yet, those laws were still passed. That's because although we could do everything ourself, it's a lot of work. One benefit of government is to make things easier. It's easier not to be tricked or misled if it's illegal to do it, it's easier not to get on heroin if it's illegal.
I don't want to always have to mentally round up, and preserving the need for it doesn't help people. This isn't the bill of rights we're talking about here.