It's written in the stone. If they chose to penalise Lewis then they could only give a drive-through penalty or, if after the race, a 25 second penalty. I've no idea if an appeal will allow a more flexible penalty to be applied, so it may get reduced (or increased) yet, but the stewards could only give a drive through or 25 second penalty if they adjudged him to have gained an advantage through going off the track.
I said on page 1 that the penalty was too harsh for the crime, so I do agree, but it's not something they can adjust (at least during the event). Perhaps that is something which could be addressed at the end of the season.
I've already explained several times in this thread as to how he could have avoided the accident (again, check the onboard to see how little he did attempt it), chose an option few have all weekend, and how he profited from it, so I'm not going to go over that again unless a new point is raised.