Integrating Iceaura's wonderfull post about empathy into my explanation perhaps i can make a more general one (without said exceptions as mentioned)
Now, we create these rights based upon soley our emotional views of how things should be.
We feel stronger towards things that display more humanly traits, and less towards those that exibit less.
We can also see thigns as "everything is chemical reactions", pain and such are simply reactions, nothing real and thus true 'rights' are simply illusions (religiouse people will of course dissagre, saying their god(s) dictated some absolute truth, however in reality we have no idea if there even is an absolute truth of anything, much less what it could be). Thus in our empathy we strongly dont want things that hurt us to happen to other humans, and then decreasing with animals.
Thus, in the case of your spider Chi, one can say that the person who says to put it out of its misery is empathizing with it, and wants to end its 'pain' in order to end their own, it is unclear wheather the spider really does experiance pain.
This can apply, more so, the more complex the creature is.
Now, then, we may kill some things because they create an emotion of anger or something (or perhaps pleasure for some...)
Say a mosquito bite me, or is attempting too, the little empathy I have for it is easily overcome by anger, or at least a desire not to be bitten thus i kill the bugger.
For other things like say mass breeding for food (pig farms, etc...) the desire for meat (at a lower cost) out weighs the empathy towards said animals, especialy if the majority of the buyers dont have to see them (and the corporations want money) of course, for vegaterians, and terrorists (err.. PETA) this may not be the case.
Now as for the legal aspect, to my knowlege one cant kill fish just because (indeed unless you have alicense, catch-and-release is all you can do legaly) Nor could one kill a bird easily unless it were near death, (they fly) unless you had a gun, in which case it would be illegal to discharge it (all subject to where one lives of course, I: in Canada)
As for Dogs and Cats, most belong to a household and thus can be seen as their owners propery (destruction of property is ilegal ofcourse) and the other few strays;
1. corpses are too big that thier decomposition would be horrible if it were legal to kill them
2. There is generaly no need to inflict violence upon said animals, and they can be more easily enforced than others (nightmere to determine if that fish died naturaly, got hit by a boat, or suffered Chi's almighty punch).
Thus birds, fish, insects any other smaller animal; it is too impractical to enforce many laws protecting them, resources are better spent.
Now, The benefits of said species on the ecosystem is a good point, as we want to live and thus must protect our ecosystem; however it is already illegal to kill endangered species, and normaly things that one might kill like, spiders and other bugs, breed so much that killing them in the amounts the average person does would not amount to anything significant. No need to protect them legaly.
Of course the question 'Is this just?' is a good one, and can of course be debated ad nauseam, and I doubt things in the future will improve the situation to quikly (I dont like spiders, so i will kill them, i like bacon, so i will support mass pig farms.)
This is I think how the majority of people see things... not that that makes it moraly 'right' and indeed, the future populace is usualy disgusted with the past's values (slavery is a good example i think, and womens rights.) Someday I do hope the bacon will be grown as simply bacon tissue in factories; if genetic engineering is allowed to advance.
-Andrew