I admire them because they certainly considered the pros and cons of doing so, and decided that they feel secure enough in their market position and prefer to give this back to the open source community
Pretty much every modern company safes tremendous amounts of money from open source (Linux and upwards in the stack), and so the OSS community should rightly be considered a STAKEholder.
The share vs stakeholder obsession in the space of large companies and corporations represents a lot that's wrong with our current markets.
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Also, the only upside to open sourcing this is getting other involved in development. I just tested on 10k images, and the promise both on compression rate and bit parity after decompression holds true.
Seems to be a pretty stable product, so the that motivation is probably only miniscule.