Question about MS/YHOO and trading regulations

Microsoft has an offer on the table to buy YHOO for about $31 a
share.  YHOO is dragging their feet and in the mean time YHOO's stock
price is hanging around in the high 20's per share.  If MS is going to
acquire YHOO, could MS just start buying YHOO stock on the open market
and save themselves a few dollars per share if they are going to end
up buying it anyway?  Is MS prohibited from doing this by some kind of
regulation?

(I realize that if MS decides to rescind their offer they wouldn't
want to be sitting on a bunch of shares of YHOO they now no longer
need but if they are serious, and I think they are, this would only
serve to emphasize the fact that MS is serious about buying YHOO's
shareholders out.)