Unsubscribing from Observables inside services?

When looking at code examples, I often see the case where observables inside services are not unsubscribed from.

Here is an example:

export class AuthGuard implements CanActivate {

    private isLoggedIn: boolean;
    private isLoggedIn$: Observable<boolean>;


    constructor(private authService: AuthService, private router: Router) {
        this.isLoggedIn$ = this.authService.isLoggedIn();

        this.isLoggedIn$.subscribe(res => {
            if (res) {
                this.isLoggedIn = true;
            } 
            else {
                this.isLoggedIn = false;
            }
        });
    }

    canActivate(route: ActivatedRouteSnapshot, state: RouterStateSnapshot) {
        if (this.isLoggedIn) {
            return true;
        }
        else {
            this.router.navigate(['login']);
            return false;
        }     
    }
} 

Is there a reason why you would not unsubscribe from the observable this.isLoggedIn$ in this case? Or is the above example just bad coding leading to memory leaks?

When looking at code examples, I often see the case where observables inside services are not unsubscribed from.

You are looking at a bug.

Is there a reason why you would not unsubscribe from the observable this.isLoggedIn$ in this case?

If you wanted to leak memory.

Or is the above example just bad coding leading to memory leaks?

Yes, it's leaking memory. The subscribe function will continue to be executed after the object referenced by this is destroyed. It might never happen if this object is never destroy, but it's an example of code that would fail a unit test where objects are created temporarily.

Objects tagged with @Injectable() often behave like singletons or have weak references. It might be working for a while, but once you use it in a temporary case it'll leak memory.