as you can see in the picture?
I think this is because they 're using ERC-20 USDT standard. So they need to pay fees in ETH ecosystem, which is predictable high. if they would use TRC-20, there would not be such problem.
$19 is too much while the base is $1.78 at high priority 20 gwei per transaction in USDT transfer. Perhaps you could contact their support and see to it if they are making a mistake about that. Maybe they are and there could be ways to fix this mistake.
1.78 is practically 10 % of 19 $. So yeah, this is logical why they don't allowed everything less this amount. I would even tell that it would be more right to increase minimum amount in this case, up to 30-35 $