In the UK there is a IR35 law which basically says if you are a contractor working for BigCo every day, 5 days a week, for 6 months we will treat you as an employee and tax you accordingly.
It is frequently got around with pretty slim "contracts" and generally the HRMC keep it in their back pocket for rainy days
I suspect the IRS will let an awful lot of acqui-hires go through so that they can pick off the obviously dodgy ones without finding them selves bogged down in masses of law suits with defensible "we really were bought out" and the horror of some bad precendants.