oops, forgot about this thread.
Aside from the cited rules violation? Because that is the single most fun-destroying interpretation you could have.
On the contrary, it can add to the fun.
The OP asked what do we do? So, even though veg had already posted the RAW, I am, obviously, expressing my opinion.
IMO, this is one of those situations where the DMG should be treated as a guide
if the DM can handle it.
Which means that NPCs (or PCs) are always prepared and surprise never happens. Which is unrealistic, shatters the willing suspension of disbelief, and continually interrupts whatever the PCs or NPCs were doing. Nothing gets accomplished and tension is higher than in a game of Paranoia.
That would be because you would be playing unrealistically. I play a realistic game. Reality says that PCs must face the consequences of their actions.
Way back in 1e, before quick draw and the like, you could not attack unless you had a weapon drawn (an action). Ironically, I believe it was called
readying a weapon. So players would start to say "I always have a weapon drawn." It was completely legal. I, and other DMS, rather than say you can't do that, would say "fine, but you suffer the consequences." So approaching potential friendlies with weapons bare might mean they would have an unfriendly attitude or even simply attack. Also, constantly carrying a weapon drawn is fatiguing so players might suffer some sort of penalty.
Likewise, in 3.5, it is completely legal for a player walking through a dungeon to declare "I'm constantly making spot and listen checks." This means that in every 'round' they have the potential of discovering hidden monsters, e.g. at the earliest possible opportunities. Yet, constantly being on the alert would realistically mean the player becomes edgy, paranoid, and doesn't notice things that are there, or starts seeing things that aren't there (which, BTW, can be a lot of fun for the DM!). The player must suffer the consequences of his actions.
So, in the examples you mentioned, the player(s) must suffer the consequences:
At worst you get the following:
"I move down the street and ready to fireball the first person that acts hostile. I do this every round."
Fine, I employ the OK_You're_Being_Silly Rule.
"After walking by several doors, a door opens and you think you see someone with a weapon coming out. You fireball. Oops it was a baker!"
Or, "oops, it was nothing but you just wasted a spell."
Or, "oops, you thought that someone was attacking, oh, and too bad the door was only 15 feet away. What's the area of effect of fireball again?"
"The monster Readies to pounce the first person through the door."
Hopefully, no DM is that ridiculous, so it wouldn't apply. Another way to think of it is that monsters aren't as stupid as PCs.
As I mentioned, I'm of the opinion that DM's allow readied actions all the time whether they realize it or not.
1) You tell the PCs that they have to cross a rickety bridge above a lava pool. If a PC blithely walks across the bridge, it may collapse and the player gets a reflex save to either jump forward or catch ahold of something and not die. Now a player who's paying attention says "if I think the bridge is about to collapse I'm going to jump forward." Essentially, a readied action. How do you adjudicate it? Hopefully, you would reward the player paying attention. You might give him a circumstance bonus to the save when the bridge collapses. I prefer to give him a perception check to notice that the bridge is about to collapse and let him take the readied action. If he fails the check, then he has to make the normal save. (So he essentially gets two chances)
2) The PCs hear some orcs. They burst into the room and get, per RAW, a surprise round on the unaware orcs.
Now, let's reverse it slightly. The PCs have killed the orcs and suspect, but are not aware of, that reinforcements might be on the way.
Combat is over. A couple of PCs wisely declare (as it indicates in the PHB) that they will cover the door and attack anything that comes through - a readied action (note: outside of combat). Now the reinforcements are aware so you should give them a surprise round per RAW. You could, of course, give the PCs a listen check to become aware, but if they fail the orcs would still get a surprise round. Now I prefer simply to give them the readied action so when the orcs burst through the readied PCs get to act. (The thief searching the bodies, however, would be surprised.)
3) The PCs have some mechanical device they have to solve (not a combat). The only way to solve it is for PC1 to do
this and PC2 to do
that immediately. The answer? PC2 readies his action for when PC1 is finished with his action.
The easiest way to handle players being silly is to allow them to be silly.
"I cover that door!"
"Okay, nothing comes out in the first round. Now what are you doing."
"Umm, I cover that door until someone comes out."
For the next ten minutes, you bore the hell out of the players by describing each and every leaf on each and every tree while making them make listen/spot checks on anything that moves. That usually fixes them.
So basically, I make the players suffer the consequences of their actions, especially unreasonable ones. Now if you want to ready an arrow at the door the thief is trying to unlock, sounds fine to me. If a particular part of the dungeon looks like a good ambush spot and you want to make a few spot checks, fine with me. If it's reasonable, fine with me.