MS is publicly traded. Wall Street obviously thinks it's a bad idea. The MS board / shareholders get to vote on it, regardless of what Wall Street thinks; but Wall Street remains the ultimate arbiter. If Wall Street continues to think it's a bad idea, they will start hammering MS stocks, which results in less value to shareholders - which is exactly the opposite of what MS is obligated to do: increase shareholder value.
The value of A/B's stock right now is mostly immaterial. Usually in a buyout, the offer is already on the table, if the "being purchased" stock tanks between now and then, it won't really affect the buyout price to the "being purchased" shareholders.. it just results in lower value to the buyer -- the current A/B stocks all get bought out and they just go away. The value of that is then to be reflected in MS stock price; if Wall Street likes it, that price goes up and MS gains the value of that purchase, and if it works out like you hope, your stock value went up by more than the purchase amount and that pretty much covers the purchase. If Wall Street thinks you just royally F'ed up, your stock price could also go down, and you just got double whammied -- not only did you pay out for that purchase, but you also just lost a lot of your stock price on top of that, so you essentially get to pay twice for a bad idea.
Now, I'm speaking of Wall Street like it's a unified decision-making body. It absolutely isn't -- it's entirely irrational and driven highly by emotion on a good day. But it absolutely does determine what publicly traded companies do, as little sense as that makes most of the time. And I'm also speaking about stock prices like the value is static; they are always to be considered over time; even if the post-purchase price goes down immediately, if MS does some brilliant things with the new acquisition, they could be rewarded over time - and shareholders will obviously consider that when they cast their votes.
And if MS does back out of the deal - there is almost always some sort of cooling off... MS can't drive this into "obviously bad idea" territory just to tank the A/B stock price, cancel the deal, then swoop back in and take advantage of the new lower stock price. Other parties; however, can, so long as they did not collude with Microsoft for the purpose of taking advantage of that. But if the deal gets ganked from Microsoft, I don't know that many other buyers would necessarily be lining up; there's not been a lot of positive news coming from A/B lately to make me think it's a good investment as an operating company. It has a lot of high value wrapped up in some old IPs, but that's not the same thing as if they were shipping huge hit titles year after year. They've spent a long time coasting on some older good games and what amounts to the annual rehash of CoD, and seem to have whiffed on eSports.