While in hiding, why did they have to apparate on the top step of Grimmauld Place entrance (and not directly inside)?

Question

As Dumbledore once explained Harry, people usually apparate in front of the house (or around the house, at least not in it) they're visiting for two reasons:

• it's rude to apparate directly into your host kitchen
• most houses have anti-apparating charms

"Professor, why couldn't we just Apparate directly into your old colleague's house?"

"Because it would be quite as rude as kicking down the front door," said Dumbledore. "Courtesy dictates that we offer fellow wizards the opportunity of denying us entry. In any case, most Wizarding dwellings are magically protected from unwanted Apparators.
(Half-Blood Prince, Chapter 4 Horace Slughorn)

But we know it's possible to apparate inside Grimmauld Place: George and Fred used to apparate from one room to the other:

“We have,” said Hermione quickly. “We’ve been decontaminating this house, it’s been empty for ages and stuff’s been breeding in here. We’ve managed to clean out the kitchen, most of the bedrooms, and I think we’re doing the drawing room tomo — AARGH!”

With two loud cracks, Fred and George, Ron’s elder twin brothers, had materialized out of thin air in the middle of the room.
(Order of the Phoenix, Chapter 4, Number Twelwe Grimmauld Place)

And about the politeness... When only Ron, Hermione and Harry are in the house, is it really rude for one of them to apparate inside the house (or even for Lupin)? Is it worth taking the risk to apparate on the stair, sometimes stumbling and letting the Invisibility Cloak slip, while Death Eaters are watching the place?

Lupin explaining he "had to" apparate on the top step, and telling about the watching Death Eaters:

“Yes,” said Lupin, “but we’re all being watched. There are a couple of Death Eaters in the square outside—”
“We know—”
“I had to Apparate very precisely onto the top step outside the front door to be sure that they would not see me.
(Deathly Hallows, Chapter 11, The Bribe)

Harry almost being seen by the Death Eaters while coming back:

Meanwhile, inside number twelve, Harry had just entered the hall. He had nearly lost his balance as he Apparated onto the top step just outside the front door, and thought that the Death Eaters might have caught a glimpse of his momentarily exposed elbow.
(Deathly Hallows, Chapter 12, Magic is Might)

Then explaining about it to Ron:

“They nearly saw me coming back in just now,” Harry said. “I landed badly on the top step, and the Cloak slipped.”
(Deathly Hallows, Chapter 12, Magic is Might)

Answer 1

The simplest explanation is that one can Apparate within Grimmauld Place, but not to or from the building. There is precedent. In Harry Potter and the Half-Blood Prince, when the students are taking Apparition lessons:

‘As you may know, it is usually impossible to Apparate or Disapparate within Hogwarts. The Headmaster has lifted this enchantment, purely within the Great Hall, for one hour, so as to enable you to practise. May I emphasize that you will not be able to Apparate outside the walls of this Hall, and that you would be unwise to try.’

_{Wilkie Twycross, Harry Potter and the Half-Blood Prince (Chapter 18, Birthday surprises)}