Because if you are not careful, you might lose your money, and those platforms have rights to reject your deposit returns when you fraud them, or violate their rules in ToS.
Don't spread this kind of hoax please, by doing that you're making yourself accomplice of scammers.
No, they've not the "right" to confiscate your goods just because you didn't respect their ToS.
When you deposit your cryptos on platforms, you don't sell them or give them for free to the platforms, it's still your property, not the property of platforms. So if they confiscate your cryptos it's a theft.
ToS have to respect the laws of the country of the platform, and I don't think there are many countries in the world where you can seize the goods of your customer just for not having respected a clause of your ToS. So such clauses are unlawful.
Moreover platforms regularly change their ToS without giving notice to customers, so customers are often unaware of the changes in the ToS. No need to mention that crypto platforms generally don't care of their own ToS when a customer ask something from them...