there are rules for attacking objects, however powers have their valid targets listed
<snip>
you still haven't shown where in the rules powers without "object" as a valid target should, (barring DM fiat or houseruling until PHB2/DMG2 came out) be assumed to have an effect on an object.
<snip>
Do the rules of a power, when using it supersede the general rules of the game? Is there anywhere before PHB2/DMG2 that powers without "object" as a target are noted as generally affecting objects?
4e can't have it both ways, it can't create a format where it lists specific targets for clarity, designate "object" as one of those valid targets and then not list it but expect it to be assumed as a valid target. What's the point of listing valid targets then?
<snip>
In essence you are house ruling it in and since other posters, including myself have given examples of DM's who actually followed the rules for powers to the letter, if you're assuming every DM house ruled this stuff in you are wrong
There is no [fire] spell in the PHB that includes "object" as a target. From which it follows that either (i) the rules on p 66 of the DMG about setting paper alight are pointless, or (ii) the DMG assumes that a GM can adjudicate the effect of a fireball upon a pile of paper. If people infact played with GMs who didn't read and apply p 42 I don't know what you want me (or the 4e designers) to do about that. I guess some people think it's "house ruling" to read and apply the rule book, and is "following the rules to the letter" to ignore the bit of the rulebook that says "Here are the charts, tables and guidelnes you will need as a GM to adjudicate actions that the rules don't otherwise cover".
As to the purporse of the targetting rules: it seems to me that they differentiate between those targets where the player does not need the GM to adjudicate fictional positioning - because by default 4e the creatures in an AoE will be marked by tokens on a map - and those which the GM does adjudicate pursuant to pp 42 and 66. (It's also fairly obvious that the main function of the target line for melee and ranged attacks is to tell you how many targets in the case of multiple attacks; and for AoE is to allow the creatures/enemies distinction to be drawn.)
Seriously, anyone who reads the rules for a system that (i) tells you that [fire] effects include explosive bursts and ignition, (ii) has rules for the vulnerability of paper to [fire] effects, (iii) has rules for resolving actions that the rules don't otherwise cover, and (iv) uses the "creature" designation primarily to distinguish indiscriminate attacks from "enemies" only ally-friendly attacks, yet concludes that a fireball can't set paper alight, isn't someone I want to game with. Is this the same sort of person who assumes that PCs can't whistle because there's no whistling skill?
EDIT:
The personal jist of my argument is that since it's unspecified whether or not area effects affect objects, that leaves the narration up to the player
<snip>
Thus, in the generalized "creature" case, I would say that the DM has the authority to include objects as affected if it fits within his conception of the power effect, while still subject to the rules constraints on damaging objects. (No psychic damage on the door). In the "enemies" case, the player has the ability to frame his power to affect objects as fits his conception of the power effect.
I think the default 4e approach - judging from both the p 42 guidelines and the description of the DM role in the PHB - is that it is the GM who gets to adjudicate rather than the player, but having regard to the spell description.
So I agree that fireball clearly targets everything in the area - as you say, that is the point of the "creatures" rather than "enemies" in the target line.
In the case of an "enemies only" fire attack, I think the GM would be within his/her rights to (say) ask for an Arcana check to hit an enemy without striking the pile of paper on which s/he is sitting. (And when one of the PCs in my game used Fire Shroud (? low-level enemies-only wizard burst attack) to take out some undead spiders running across the shelves of a library, I
did ask for an Arcana check to save the books.)