Milk is good for ponies.
Milk is especially good for very young ponies.
Milk's crucial for their development at that age, really.
But the way Big Mac's carrying on about things, you figure that Applejack would be feeding cyanide to baby Applebloom, instead of milk.

Clearly, the guy needs to stop acting embarrassed and start acting like a responsible parent.
It's not like there's anything immoral about a mother nursing her child in public, right?

Right?

I'm not sorry about this.

Story is possibly in-universe; your interpretation of whether this is alternate universe or not depends on who you believe Applebloom's parents are. Either way, I'm pretty sure this gag wouldn't work without this particular assumption, So I offer my apologies to people who don't like the concept.

Oh, and rated teen for the lactating bits of female anatomy.