It's pretty straightforward - find a high-water mark for customers in terms of their inbound data, that 99.9% fall under, say, for linode, 99.9% of their customers pull in less than 16 megabits/second sustained, which is 7.2 Gigabytes/Hour, 5 Terabytes/Month.
And then explain to your customers that 99.9% of them use less than 5 Terabytes/Month bandwidth, and that effective now, the first 5 Terabytes/month of bandwidth is included in the cost of their VPS, and for those 0.1% of customer who exceed that 5 Terabytes, they'll get a very reasonable rate of $0.01/gigabyte ($10/Terabyte) for inbound data in excess of the 5 Terabyte initial allocation.
Realize that linode is probably paying less than $4/megabit @ 95th percentile - but the vast majority of that is likely downloaded, so they have an asymmetric allocation that they are trying to utilize with this "unlimited inbound" offer.
I just want the HN audience, in particular, to realize that the statement ""This means you can upload an unlimited amount of data to your Linode without having to pay for any of the incoming data transfer."" should not be taken at face value if you are putting together a business plan.
In general - bandwidth is priced @95th percentile on the internet, and any time you run into the phrase "unlimited" you should take it with a grain of salt and get something in writing before you rely on it.