Apart from the legal reasons that may have forced them to do so, they also got other people contributing to the codebase and fixing bugs. Other companies can also use the component, which is a great thing for a rendering engine - it will get more popular and websites will have to test and optimize for it.
Regarding iTunes, Apple wouldn't want anyone to release a competing program. You could probably outsource some common libraries/components, not sure if this is done. But anything specific will not be used or improved upon by others anyway, so there would be no benefit in opensourcing it.
Regarding iTunes, Apple wouldn't want anyone to release a competing program. You could probably outsource some common libraries/components, not sure if this is done. But anything specific will not be used or improved upon by others anyway, so there would be no benefit in opensourcing it.