[sokoloff]

You might not be allowed to sell the shares per an insider trading policy or per a disclosure policy ("CEO of XYZ dumped 50% of their shares" looks way worse than "CEO of XYZ borrowed $25MM to build a luxury house.")

You might not want to sell shares that you've held 306 days, preferring to hold them an additional 2 months to get long-term capital gains treatment on them.

You might not want to take capital gains (even if long-term) on this year's income tax. Maybe you want to defer it to January; maybe you want to defer it to a later year when you expect to have a lower capital gains bracket or when you expect to be able to avoid the Obamacare surtax on investment income (via repeal or via lower AGI)

You might not want to sell shares if the margin loan rate is lower than your expectation for growth of the shares.

[beagle3]

The first one is likely still illegal under SEC rule 144 - they don't enforce it often, so it isn't very well known, and it's also a little vague, so hard to say whether it applies in this case - but it Might.

[ksec]

Never been to US but everytime i read anything like this it sounds like US has the most complicated Tax system in the world.

And you have to do it yourself!