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Something like this should just be released as an open source lib.

Why? It's the developer's choice to sell it or give it away for free.

They wrote something, they noticed a lot of uptake and now want to see if they can monetize this. I don't see a problem here.

Isn't TinyPNG a web GUI around pngquant command line utility?

Probably some AdvPNG &al as well, on top of pngquant.