[the_why_of_y]

You can bundle LGPL libraries with proprietary applications quite easily; it just has to be possible for the user to replace it, which is rarely a problem because the usual situation is that your proprietary code sits "above" the LGPL library.

But reportedly Apple does not allow LGPL in its app store for some reason.

[gillianseed]

Well there are likely lots of potential markets where a prerequisite of allowing the end user to swap out LGPL libraries won't be accepted (sadly), not only Apple but the consoles as well, so from that perspective Unity needs to buy a license which allows them to forego the LGPL requirements.