I can see your point, however this is just my opinion on the spawner situation;
Pigs are the cheapest, easiest to farm and rack up a lot of income per double chest. Technically cow spawners would give you more money per chest, considering the price of beef AND leather. Similarly for sheep, they have mutton AND wool. Lastly, chickens while being relatively difficult to farm automatically, they have chicken AND feathers.
My point being, if the prices of the drops for the other mobs were buffed, it’d just be way too OP. Currently at the moment, it’s way too OP with pig spawners, but that’s just my own opinion as traditionally mining should be the main income on prison servers, and it’s just how I personally prefer to play. However, I’m not totally against mob grinders. For the other spawners however, taken into account that they have multiple drops, they already make significantly more than pig spawners, buffing it would only make it more drastic. Reason people prefer pig grinders is because they’re easier to farm, easy to make, cheap(er) to buy, and the income they bring in is something like $1.3mil per double chest.