I'm sure the Mother Box was healing Batman, but Batman explicitly stated the beating was for Darkseid's amusement. Considering that he needed Batman to get the codes Bruce changed or Apokolips would be in ruins, I highly doubt Darkseid was hitting him all that hard.
__________________ Bluewaterrider: "I'm surprised that a Skyfather like Zeus defeated Hulk when Zeus' Top-Tier son Hercules has lost to Hulk."
Conversely, Darkseid, knowing those Apokolips "Parent" Box devices better than anyone, may have had little if any fear of doing damage to a man who knew was protected by one.
As alluded to before, they seem to be fairly powerful:
A cubic foot device generating enough power to affect the space/time continuum?
Change an entire STAR to a different life phase (yellow sun becoming a red sun from the Father (Mother) Box explosion)?
I think Darkseid should feel comfortable enough to let off some steam when THAT kind of technology is protecting his opponent.
To be fair, though, I myself argued largely as you did a while ago, and Darkseid punching hard enough to let off steam or gain amusement doesn't make necessary Darkseid splitting a planet in two with every blow:
If Mother Boxes provided enough protection to let Darkseid hold little if anything back while beating on Batman, then Orion should have nothing to fear from a hand-to-hand fight with Kalibak, Superman or Darkseid.
__________________ Bluewaterrider: "I'm surprised that a Skyfather like Zeus defeated Hulk when Zeus' Top-Tier son Hercules has lost to Hulk."