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.
