Open source software only comes in one edition: awesome.

Alright, I can come up with a reasonable explanation:

For databases and servers a lot of bugs and corner cases are only hit by servers that get loads and loads of traffic. Fixing those bugs cost a significant amount of money. By charging more to the high-traffic servers the cost of fixing those bugs can be justified.

It is also fair, in a sense, because only the high throughput servers really benefit from the extensive testing and stability, so why shouldn't they be the ones to pay for the fixes?

Like I said, market segmentation in software does not necessarily have to be a bad thing. Consider a hypothetical text editor that comes in two editions. The "basic" edition has features like syntax highlighting, regex search and replace, etc. The "advanced" edition has a feature that allows you to open and save files not just locally, but also via SFTP. When it was first written, the editor didn't have the SFTP feature, the author developed it because there were some users who asked for it; they weren't the majority, but the author thought the idea was nice. Instead of incorporating that feature into the one and only edition, the author decided to offer two editions: a cheaper "basic" and a more expensive "advanced" edition. It was the author's idea of placing value on the effort involved in writing the SFTP feature. The majority of users can go on happily using the "basic" edition without paying extra for it, while those who really want SFTP can pay for it.

One way of looking at this market segmentation is to say that the author was greedy and imposed an artificial limitation on one of the editions in his product. Another way of looking at it is to say that the author offers two products that satisfy different needs at different costs. If we were talking about two products from two different authors, we would be comparing their features and prices quite naturally, without complaining about market segmentation.

However, when you're offering exactly the same product, with no differences, for two different prices, then you're just being greedy and taking advantage of your customers (which is something commonplace nowadays and most people, myself included, just accept it). Close to that is the situation in which you slap a truly artificial restriction on a cheaper edition of your product, not through a lack of feature, but through a deliberate crippling of that feature (e.g. the five-dollar edition can open only up to 3 files). The exception, of course, is a crippled free edition which allows potential customers to try your product before deciding whether to buy it. In that case, you're just being annoying ;)

That, in summary, is why I believe that market segmentation is an advantage, within certain ethical bounds.