I would like to preface that I am not complaining about the bans, I just came to a realization as a consequence of this conversation and would like to share it.To answer this, OM's current tiering policy dictates that Pokémon bans are prioritized over move and ability bans in the case when only a sole donor is present for the element. This is why Lilligant was axed and not Victory Dance.
As for Populatiom Bomb, in that case it would require us to ban both Maushokd and Tandemaus, which at that point voids the distinction, so we just ax the problematic move.
It may not be pleasing to you, but that is how it is, and we will be following OM Tiering Policy.
The universal rule of the OM is the following: " Pokémon gain access to the abilities and Movepools of all Pokémon that share their typing." which, technically isn't true. Rather "All pokemon of a shared typing have access to a specific pool of moves and abilities". This second definition comes from the idea that the OM being played is it's own game in a vacuum based on this AAA post. The first one is necessary as an explanation relative to the existing series of Pokemon we all know, so we can understand the OM and how it works, and it's a very good one at doing exactly that.
The way the OM works mechanically, is if you are of type A then you have access to moves B and Abilities C. Any Normal type gets access to Population Bomb, period. Even if Maushold was the only Pokemon that got Population Bomb, banning Population Bomb and a random Normal type with objectively worse offensive stats than the main abusers would be against tiering policy; The former is broken, and is therefore banned, but the latter is an extremely arbitrary ban. While Lilligant-Hisui was clearly banned due to having "donated" Victory Dance as mentioned in the original banning post of Lilligant-Hisui. Same principle applies to abilities which, at the time of posting this, Komala is legal, but Comatose is banned, which, in conjunction with the Lilligant-Hisui ban, is inconsistant against the current standard of banning "donators".
Again, not hating on the bans, I just noticed in inconsistency in tiering policy, which I know Smogon takes VERY seriously and wanted to throw in my 2 cents. The OM could just be determined by "donators" but, that is very inconsistent with how other OMs handle their tiering policies. (AAA revoking ditto's ability to use imposter, most notably).
TL:DR banning pokemon based on moves/abilities that they donate is arbitrary relative to the mechanics of the OM.