You are establishing one rule for your game and I'm establishing another. Our two rules are different from each others house rule by virtue of the different thresholds.
But deciding that (say) the scroll is burned along with the wizard who was holding it
isn't establishing a rule at all. It's applying a rule (or, rather, a conjunction of rules: I've cited the relevant passages of the SRD upthread).
The fact that you might construe the rules differently doesn't change the fact that what I'm doing is construing and applying the rules of the game.
If what you decide only applies to your table, it is a house rule.
But
anything that I decide applies only to my own table.
That's not a point about the content or source of rules, though. It's a point about authority. Unlike in some other systems of rules (eg legal systems administered by court hierarchies) I have no authority to make my construction of the rules binding on anyone else.
In circumstances like that it's natural that varying interpretations and table practices will flourish. That doesn't mean that everyone is house-ruling, though. That just shows us that even reasonable minds can differ over the interpretation of natural language rules and over their application to (imagined) empirical circumstances.
At least in common law countries, judges disagree all the time over rules that are (generally) drafted with far more care than the D&D ones, that have received far more skilled analysis and commentary, and in cases where the stakes are much higher and hence much greater effort is made to get it right. They are only held to some degree of uniformity by rules establishing hierarchies of authority and rules for casting votes on the bench. Why would we expect the D&D RAW to produce any
greater uniformity of interpretation and application?
You don't include something like that if it can burn everything. You either say it can burn everything flammable or you leave it alone. It's crystal clear that fireball is not supposed to burn worn or carried items.
Obviously I don't think it's crystal clear at all. I've quoted the text on p 87 that says that objects can be damaged by attacks, including spell attacks, in much the same way as creatures can. The same text also mentions that objects always fail their DEX saves. Given that a big chunk of the damaging spells that involve DEX saves are the fiery ones (Fireball (including the delayed version), Burning Hands, Flaming Sphere, Fire Storm, Flame Strike, Incendiary Cloud, Meteor Swarm, Wall of Fire, maybe some others I'm missing*), that text would be fairly pointless if those spells couldn't damage objects.
Of the spells I've mentioned, three have no text about damaging objects (Flame Strike, Incendiary Cloud and Wall of Fire): it would be odd if that absence of text meant that they
could damage worn/carried objects though Fireball, Burning Hands and Flaming Sphere cannot. Delayed Blast Fireball, Fire Storm and Meteor Swarm all have text noting that the spell damages objects in the area, as well as igniting objects that are neither worn nor carried. Few other spells have such text: does that mean that the passage on p 87 is meant to be applied only in the context of 7th and higher level spells?
I think the text on p 87 is clear: the GM adjudicates damage to objects caused by spells, and - given that text - there is no general presumption that spells don't damage objects. (Nor is there any general presumption that weapons don't damage objects: a GM who narrates a slash in the clothes of a hobgoblin who is cut down by a PC's sword is not houseruling either!) Some spells have text that, as far as they are concerned, eliminates at least some elements of GM adjudication by stipulating outcomes for certain classes of objects.
[size=-2]* In the SRD, the following spells do damage and involve DEX spells but don't deal fire damage: Acid Splash, Black Tentacles, Blade Barrier, Call Lightning, Chain Lightning, Disintegrate, Ice Storm, Sacred Flame, Storm of Vengeance, Wall of Ice, Wall of Thorns - that's 11 spells, compared to the 9 I've mentioned above (and Lighting Bolt also has the text about igniting flammable objects that are neither worn nor carried). Explosive Runes and Prismatic Spray/Wall have both fire and non-fire options.[/size]