1) Why oh why do fundamentalist Christians use the absence of something in the Bible as grounds to condemn it?
Really? Fundamentalist Christians condemn the bible? I can't recall having ever done so since becoming a Fundamentalist Christian....
2) Some church practice, however inconsequential, even something like wearing robes or lighting candles, is attacked as "unbiblical".
Well, because some of my fellow Protestants (not just Fundamentalists) have not bothered to consider the difference between "unbiblical" and "contrary to Scripture". Where in the bible, for example, does it instruct us to build edifices to be used for the sole purpose of worship? Where does it tell us to pray with palms together, or with arms outstretched, or with fingers interlaced? All of these things - and many, many more common Christian practices - are "unbiblical".
3) Unless the Bible is specifically talking about an issue, you can't logically use the fact that it's NOT MENTIONED to condemn it. It makes no sense.
Agreed - though, along the same lines, we can't logically condemn such a belief (that only what it in the bible is valid Christian religious practice). If someone wants to believe that only doctrines and religious practices described in the bible are appropriate, that's fine - as long as they don't condemn other Christians for following beliefs not contrary to Scripture. Paul addresses this specifically. In his example, he speaks of Christian vegetarianism. His command: to eat as Christian vegetarians do when dining with them so as not to offend their "unbiblical" but acceptable Christian beliefs.
4) Another example: some use this to condemn infant baptism on the grounds that it's not mentioned. Guess what? Specific mention of baptising the elderly isn't mentioned either. Baptism is discussed as a general practice; there is no need to break people down by age.
Agreed. It's interesting that in the same passage that my fellow Protestants use to justify adult-only baptism, Peter is speaking to a crowd of men! Are women, then, also forbidden to be baptized because Peter did not address them specifically?