Swift协议获取者和设置者

为什么Swift自动允许开发人员设置属性值,即使protocol只声明了getter而没有setter。检查以下代码:

import Foundation

protocol Pedometer {
    var pedometerAvailable: Bool { get } 
}

class MockPedometer: Pedometer {
    var pedometerAvailable: Bool = true // even though the Pedometer protocol is only getter I can still set ??
}

let mockPedometer = MockPedometer() 
mockPedometer.pedometerAvailable = false // why I can set the value here