I don’t understand how you are confused. If I need to check if my app works on m1, I can now spend $24 to do that instead of $650 or whatever it retails for.
If you're already in AWS, your deployment infrastructure and tooling is already configured and tuned for AWS. Deploying a Mac EC2 instance is just adding another target. Whereas deploying to another provider is an unknown amount of work to integrate with their API (if they even have one). The wall-time cost of people's time and effort vastly outweighs the marginal savings you'd get by going with a low-cost provider.