McGonagall ashamed or fibbing?

[potioncat]

No: HPFGUIDX 113232

Something about McGonagall jumped out at me while looking for 
something else.

Let's start with canon:
SS/PS chp 14 last pages

Professor McGonagall, in a tartan bathrobe and hair net, had Malfoy 
by the ear.
"Detention!" she shouted. "And twenty points from Slytherin!  
Wandering around in the middle of the night, how DARE you--"

Malfoy protests that Potter has a dragon.  She accuses him of 
telling lies and hauls him off to see Professor Snape.(That must 
have been an interesting interview!)

chp 15
McGonagall has returned to discover Harry, Hermione and Neville are 
also up. Part of her tirade includes:  
"You fed Draco Malfoy some cock-and-bull story about a dragon, 
trying to get him out of bed and into trouble."  
cutting to:
"I'm disgusted," said Professor McGonagall.  "Four students out of 
bed in one night!  I've never heard of such a thing before!"

Having docked Malfoy 20 points for being out of bed and lying, she 
now docks each Gryffindor 50, including Neville, who she thinks was 
tricked like Malfoy was. She ends with, "I've never been more 
ashamed of Gryffindor students."

*********************************************************************

Seems the Marauders did pretty well. You'd have thought they would 
have at least been caught once or twice out of bounds at night.  

What is she ashamed of?  That they were out of bed or that they 
tricked Malfoy?  (Remember, she doesn't know the truth.)  And how 
must this have appeared to Snape? Dejavu?  

Not too much later, though I'm not sure of timeline, in chapter 16 
Snape warns Harry, "...any more nighttime wanders and I will 
personally make sure you are expelled."

I always thought Protector!Snape was trying to keep Harry safe.  Now 
I'm wondering if Snape was trying to prevent a repeat of history. Is 
he protecting Draco? And was McGonagall telling the truth? never so 
ashamed, or is she also aware of the Snape-Marauder episodes?

Potioncat