It gets even better than that if you're in an approved accelerator / incubator: you get $60,000 USD in hosting credits, at a rate of $5,000 a month. We're entirely hosted on Azure using their Linux VMs and have had a great experience with them so far.
I think the reasoning is that if you convince an accelerator/incubator to accept you your product has a higher chance of succeeding. This will mean that the chance of you still existing (and starting to pay) after 3 years should be significantly higher.
Many companies have discounts that they can afford to give, but they make it a bit of a hassle to get them, so the bulk of customers keep paying full price but they can also retain the miserly customers at a reduced profit margin.
For example, some food retailers publish discount coupons in newspapers, so the discount is only available to people willing to go through the hassle.
It's also worth pointing out that BizSpark imposes no requirements or obligations on the startup. You are free to join BizSpark and never use any of its services and you don't owe MS anything at all.
It is also worth point out you can add up to 8 developer accounts (now seems to be 4?) under bizspark programme and the $150 credit is per developer, so the total saving for 3 yrs is up to $43200 (crazy?)
Also, you can enjoy additional discounted hourly rate for up to 33%, aws/google can't compete with that.
+1 to that
Bizspark is an awesome program for startups. We are running linux on it w/o any issues
[~3 servers come free]
Best part - there are no restrictions on what s/w stack you run..
Considering that almost any IT startup can access to bizspark, it is a nice thing to at least try it.
Also I love the fact they are distributing their sdks for other os (linux and osx) and that they are all opensource on github