Heading towards CIS-2020.

it's unlikely there will be any player-enforcers who would be against screenshots anyway.

That's exactly the point. It's just that here, by general principle, it's either allow all third-party non-game stuff by default, or forbid all third-party non-game stuff by default.
And to separately write permission for screenshots, separately for a guard calculator, and tomorrow they'll invent some other nuisance. And there are a number of players (among whom are CIS Champions), and this is visible on streams, who do not allow themselves to use the same screenshots.
And at a live tournament, we'll get a conflict situation: a hypothetical losing player will say, what the hell is a guard calculator being used here, if the HeroesWorld rules say that third-party software cannot be used:
"2. The use of additional programs that speed up the game process is allowed, in particular:
- A mod to improve the functional part of the interface - HoMM3 HD,
- Animation accelerator in battles - Cranim.
The player using these programs and files is responsible for all bugs that arise from their use.
The use of any other additional programs and files affecting gameplay is prohibited without agreement with the opponent."
So we, in essence, have simply adapted an existing rule for the tournament, phrased it in such a way that no such discrepancies arise. Because try to figure out in the HeroesWorld wording — does knowing the range of Pandora's Box contents affect gameplay or not?

Added after 13 hours 2 minutes
Sibiryaks, grab it while you can.