This isn't always true.

The Bond of Blood charm - the blood charm that protects Harry and is "recharged" when he spends time with Petunia - is upon Harry, not the Dursleys house. So protective spells and charms (i.e. "wards") do not have to be static.

We also know they don't have to be only defensive, as in HBP we're told that the protections around Hogwarts are upgraded to included various anti-hexes and anti-curses. We know from OotP that anti-curses etc. are the same spells as curses, only used in a defensive manner.

 

Taure: Ah, I phrased that poorly. what I ment to say was 'Reactive' rather than 'defensive'.

I'd also like to know what the supposed 'Bonds of blood' charm actually did. Cause as far as I can tell from cannon, it did nothing to anyone other than Quirrel, and that's assuming it was the charm that did it rather than instinctive 'Accidental' magic. :-/

 

Well, Lily's sacrifice - that which powers the charm - was directed at protecting Harry from Voldemort.

So I would imagine the Charm protects Harry from only Voldemort himself. Which explains why the Dementors were able to harm him. However, it doesn't explain why Harry would be safe from the Death Eaters. Perhaps additional protective spells were placed upon the Dursley residence without the Dursleys realising?

I would suggest that the Blood Charm would consider Death Eaters an extension of Voldemort, and thus protect him that way, but this has a big problem:

After Voldemort resurrecting himself, it is him having Harry's blood which protects him from the Charm. The Death Eaters don't have Harry's blood, so if the Charm considered them extensions of Voldemort, the Charm should still protect Harry from the DEs - which it doesn't.

 

So it was focused specifically at Voldimort?