It’s not illegal. They have no contractual relationship with you, they have a contractual relationship with their client. Their interest is in paying as little as you will accept to absolve their client of further liability. Their client has damaged your property; if there was no such thing as insurance, you would be filing a lawsuit against the person who damaged your property, seeking a judgement against them. When their insurance offers you a payment, they’re offering to settle your claim of damages against their client; you’re agreeing not to seek further damages.