“A var in a protocol with only get isn’t just a let?” No. A let indicates a constant. But that is not the case here. Consider the following:
protocol SomeProtocol {
var someProperty: String { get }
}
class SomeClass : SomeProtocol {
var someProperty: String = ""
func cla () {
someProperty = "asd"
}
}
let someInstance = SomeClass()
print(someInstance.someProperty) // outputs ""
someInstance.cla()
print(someInstance.someProperty) // outputs "asd"
The protocol specifies what the conforming class shows to the outside – some property of type String named someProperty which you can at least get.
If the protocol specifies { get } your class can choose to conform via let someProperty: String = "" but it can similarly choose to conform via the above code. If on the other hand the protocol specifies { get set } you cannot use let in the implementation but have to make it set-able as well.
A protocol simply cannot define that a value has to be constant – neither should it, that is an implementation detail that has to be taken care (or decided about) by the class / struct that implements it.