Ahhh ok HB I see where you're coming from.
As far as my understanding initially LMCA agreed to "sell" back into the buyback on a 1:1 share basis so that SIRI could begin the buybacks before LMCA went to control without pushing LMCA to control before the FCC responded and gave the go ahead.
After LMCA went to control they axed the agreement to sell into the buyback on a 1:1 basis.
If LMCA continues to purchase along with SIRI it would be good, yes... but I am not so sure they will as it will inject more high basis shares. Better to simply allow SIRI to buyback from the float and LMCA stay hands off, increasing LMCA's stake.
This is, of course, assuming LMCA sees appreciation in the share price of SIRI beyond what they could get on the $1.5 billion or so locked in the 10% high basis spent to go to control. So long as the return on those shares is lucrative enough I expect LMCA not to sell back into the buyback (and as long as LMCA doesn't have a need for the cash elsewhere).
Since I don't see LMCA buying more shares, I think it's best LMCA sits back and lets SIRI's buyback push their stake up naturally. Honestly I see SIRI having enough cash flow and borrowing power to eliminate the non institutional float... currently it's only about $4 billion worth of shares at present pricing.