Whether or not the doctor has interesting decisions at night has no bearing on the scum team, which means that from the scum team's perspective, giving them some more certainty regarding their night actions working as intended is a positive.
It's clearly a negative for the doctor in all respects (increasing the number of interesting decisions, raw power level of the role), but it's not a negative for all players.
And there's no implicit assumption that people will play the doctor role poorly, the rule simply prevents one method of poor doctor play from occurring. It's a common enough happening in games with a newer community that taking action to prevent it might help the balance (preventing a poor line of play that happens a minuscule number of times in an experienced community is of dubious merit, while preventing that same line of play that happens, say, 80+% of the time in a newer community may have more merit).
FWIW when we started on webdip, I made the roleblock a factional power and allowed the doctor to save whoever they wanted except for themselves. I think that's probably the best compromise of the different tensions at play. But I'm also not involved in this game, likely won't be involved in any emerging vDip forum mafia community, and thus don't have a vested interest in any particular outcome. I'm also not really familiar with how the webDip forum mafia community evolved, so it's possible Jamie's decision to use this set of doctor rules is rooted in experiences I'm not aware of. I thought to give my 2c as someone who has a little experience in running these games and creating formats that help grow mafia communities while still giving veterans meaningful decision-making with their roles.