Because Microsoft permits testing without a license? Or because you believe, in practice, Microsoft will not assert its intellectual property rights against someone who is only testing? (I am personally OK with the first, not the second.)
If I would have to pay $100-$200 for the software subscriptions for my Windows VM (windows license, office) it gets close? Especially if Microsoft is also throwing in some hardware so my local computer is taxed less? Of course streaming a cloud desktop introduces its own problems (latency, unreliability, etc) but also advantages (portability, reproducibility, etc)
They are bundling the IaaS hardware with the price of the MS software stack subscription. You need to net out the cost of standalone windows and office from the monthly price. At the lower hardware tiers, it's a bigger percentage of the price.