Why are non-sendable properties mutable from Tasks?

A lot of online resources say non-final classes are not Sendable by default. That means classes and their properties are not thread-safe. So why is this code not raising an error:

class Counter {
    var count = 0
    func increment() {
        count += 1

class Tester {
    var counter = Counter()
    func mutate() {
        Task {

I was expecting an error to be raised at counter.increment() since counter isn't Sendable.

Can anyone tell why this code isn't raising an error?

If you set the strict concurrency checking flag, you'll get a warning here. Because of that, I'll assume this is by design and not a issue. Perhaps to support older code that lives outside the concurrency paradigm by default (?).