Open source software is wonderful and I love when companies create and support FOSS. That said, to say it's immoral not to give away the work you produced/paid to produce is ridiculous. You're not taking anyone freedom away by providing proprietary software. As I believe another commenter replied, they're free to not use it or develop the software on their own.
If it's too difficult for them to replicate or for a competing FOSS product to exist, then it probably is complex enough to warrant payment.