Simply, so they dont loose the prize money they earnt last year under the BMW Sauber name, Rumored to be arround $21.5million. And it is also so they didnt loose their space as written down by the Concorde Aggreement.
If BMW Sauber last year had signed the concodre aggreement before the Valencia GP last year, we would have seen BMW Sauber with the numbers 9 & 10 and not Williams, and been beside them in the pit lane, and not down inbetween Virgin and Hispanica this year.
As it is, its name only for one year, then they can change their name.
The reason Mercedes got away with it at Brawn is simply because it was a buyout and not a transfer of ownership that the Honda to Brawn was, thus loosing Brawn its $14.5million in prize money and standing in the pits if it were to be Honda, and it just couldnt be Honda as Honda made a change of name a requirement of the transfer of ownership to Ross Brawn and Nick Fry that cost them just £10, the standard ammount for a transfer of ownership registration and or ownership form costs to be lodged at any major Post Office in the UK.
The thing is, they can change their name mid season, as long as 50% of the season has been concluded, as McLaren did so when they changed from West McLaren Mercedes to Team Mclaren Mercedes in 2005 i think it was, and Renault last year from ING Renault F1 team to Team Renault F1. Id expect BMW Sauber to be changed to (Title Sponsor) Sauber-Ferarri after Round 10, whitch is after the British GP.