Having the HK government do Beijing's bidding rather than Air China would be a smarter move on Beijing's part. Making CX an official part of a mainland state-owned enterprise would make CX liable to potential US sanctions if CX is seen as an accomplice in violating HK's autonomy in any way.
Of course, I suppose US can see no real distinction in the move. The HK government after all does Beijing's bidding without question. So for all intents and purpose, the HKSAR government is merely an extension of Beijing's central government in Washington's eyes, hence there is no difference between an Air China takeover and a government bailout in terms of how CX's idenity and status is viewed abroad.