Apple did have to open source WebKit (the parts of it based on KHTML). WebKit was based on LGPLed code, which means if they made any changes to it, they had to open source them, or be in breach of its license. Contrast that with their kernel code which was also based on open source code, this time BSD licensed, which therefore did not have to be open sourced, and ended up not being open sourced.
http://en.wikipedia.org/wiki/WebKit#Origins
They did not have to use open source, true. they could have developed their own browser engine from scratch, but I claim they did so to save development costs, not to support open source. Feel free to claim otherwise.
And finally, I dont give a rats ass what Adobe or Microsoft or Amazon or Google or Facebook or whatever corporation you care to name did or will do. I dont condone anti-social actions simply because more than one entity does them. This thread was about Apple, but I can and do say the very same things about all these other companies.
I say that this move by Apple is bad for the ebook ecosystem, and for society that will depend on that ecosystem in the years to come. I have explained my reasons before. Those reasons stand, irrespective of whether 10 other compaines would do the same or whether "the market" approves of Apple. I choose not to outsource my ability to make moral judgements to "the market".