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.
They are not your property, though. An exchange can't even point to the bitcoin you own on their exchange, because it is all combined in consolidation transactions in to their main central wallet along with everyone else's deposited bitcoin. All you have is an "IOU" in the form of numbers in your account balance. It would be like if 50 people give me a glass of water and I pour all the water in to a barrel. Sure, I might owe everyone a glass of water, but you don't retain ownership of the water because that would be impossible, and the water I promise to give you back won't be the water you deposited.
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.
Not only can that happen, but most of the time the laws of the country encourage it. The most common reason by far that people have their coins confiscated by an exchange are because the exchange starts demanding incredibly invasive KYC with no forewarning, and blame the laws of the jurisdiction they are based in. If the users don't want to give some stranger online all the information required to completely steal their identity and ruin their life, then your coins are essentially confiscated and you can no longer trade or withdraw.