I don't understand why they are quarantining people without symptoms, as we're being told it's spread through coughs and colds, so there's no risk until symptoms develop.
Because once the symptoms appear, they could infect multiple people.
Because people are arseholes, hence why the person threatening to walk out during the quarantine period has caused the change in rules/laws.
Because people who develop symptoms cannot be trusted to quarantine themselves.
Because there will be people who believe it's all a fuss about nothing, it's all a devious plan by Big Pharma/The Government/The Lizard People, that faith in God will bring them through, that they can be the one who fulfills God's Will in bringing about the End Times/Vengeance with the disease/that if they are going to die, they're taking as many other people with them as they can (see deliberate infections of HIV for that one), that they will be taken to a concentration camp and exterminated to prevent spread, they will be separated from their partner/mother/children, etc, etc.
Because if it goes wild and does have catastrophic effects, fear will result in others attacking those who have been in contact or have symptoms.
Because the greater number of hosts means the greater number of opportunities for it to mutate.
Watch any zombie or 70s disease movie and you will get a full range of reasons why people cannot be trusted.