[fredkbloggs]

I assume your question was intended as rhetorical, but there's an actual answer to it: because when the owner of the Yahoo stock (or a hypothetical Yahoo holding company for Alibaba stock) sells his shares, he will pay tax on his gain from that holding.

To turn the question on its head: why should some capital gains be taxed twice and others once (or in rare cases, three times or not at all)?