Because as the name says, it's for developer's use. You cannot use it for production use. It's convenient for someone who may be contracted to develop databases but the company still has to pay the full cost for standard or enterprise licensing.
Just to be sure; this is by no means abberant. Oracle and IBM charges just as much, if not more, for their products, though Oracle provide a free developer edition (with same restrictions). I'm not sure about the IBM's licensing terms, but wouldn't be surprised if the terms were quite similar (e.g. a free copy of developer edition or substantially reduced fee for the same edition).
MySQL, PostgreSQL and Firebird are free because they're open source. I don't know about the other two, but in case of MySQL, one can elect to pay $500/year for "enterprise service" which provides several additional benefits and support whereas the free edition is basically a DIY solution; it's expected that you would learn to administer the server on your own, perhaps with some help of community resources. Of course, this may not be appropriate for mission critical applications where business make or lose money over its working or not working. In that case, enterprise editions (for any products you may choose) start to make more sense.
I work for a project management company which basically provide the service of developing & maintaining a business solution using SQL Server as backend and Access as front end, and that's expected that the clients pay for such services. Also, it's not unusual for a DBA (for any products) to make more than $50,000 a year. Hopefully this help illustrate the true cost of maintaining a full-on RDBMS and *why* the business has to support it. Fortunately, any business has a large range to choose how much they can justify spending on from $0 (well, that's imaginary since you still have to pay someone to administer but $0 for software licensing & hardware acquisition is mighty nice) to prices well into hundred thousand dollars.
I hope that helps.