They say it's part of the deal, if the VC says no then there is no deal and company is free to shop around for someone willing to buy the company with the preference in place.
If they are just after the team, they could just offer signing bonuses directly to the team. Such as when Borland didn't want to sell to Microsoft.
The VC's BATNA to the acquisition is owning a shell of a company, investing more money, etc. After UA takes the founders and employees SG has no founders and no employees. It's pretty much worthless.
The team's BATNA to the acquisition is a signing bonus, or another job.
The founder's BATNA to the acquisition is a signing bonus, or another job.
The acquirer's BATNA to the acquisition is a giving signing bonuses, or hiring the team.
Everyone's BATNA is signing bonuses so if the VC screws around on the preference they get nothing. That's why were not talking about a team change because the best situation for everyone is for the VC to give up the preference.
You get deals done by presenting an option that is superior to the BATNA. (Assuming everyone is rational)
See KART / Indy for what happens when people are prepared to accept the BATNA in the face of a superior offer.
If they are just after the team, they could just offer signing bonuses directly to the team. Such as when Borland didn't want to sell to Microsoft.