Google uses startups for hiring inflow, I doubt they want to choke them off.
They do this simply because that is the only way for them to either get the talent of the startup, or get a competitor to start working on their behalf.
If the startup is successful, I doubt the founders would be particularly interested in giving it up without some compensation. If it isn't successful, then it doesn't matter, since Google will be able to recruit them for far less money once the startup is dead.
It is in every company's best interest to pay the people they hire below market rate for their work. The only benefit Google gets from this is that they know the people they recruit through acquisitions are capable of building things. It's a trade-off: is it better to acquire a company, knowing the people aren't losers? Or is it better to avoid having to pay market value, at the risk of picking up a loser every now and again?