I think the distinguishing point here is what’s hinted at in the OP: it’s not the ch itself, it’s the “sch” that may force the pronounciation into bruschetta. I don’t know linguistics at all, though, so it may be wrong.
Again, and i’m not a linguist, it may be because that’s how these words are pronounced in the english language, or it may be that there is no native pronounciation of the “foreign” run on of letters (meaning like zz, gui, ci, cc).
In “correct” Italian (what you’d learn from a textbook), it’s [tʃ] as in chalk. However, there are dialects where it’s softened to [ʃ]. Central Tuscany, for instance.
What group of letters has a “native pronunciation” in English? I don’t have a link here, but I think the essay where someone spelled potato starting with a gh (as in hiccough) is from GB Shaw.
do languages not have set pronunciation rules? what i mean by “native pronunciation” is that if the letters, for example “zd” aren’t used together in a certain language, it won’t have a pronunciation for it, so you would have to use the foreign pronunciation of the word (because the word would be coming from a foreign lexicon, as “zd” wouldn’t be an english word [yet])
English doesn’t seem to have one, no. It’s one of the main complaints of ESL people, that it’s not like learning a language but like learning two. Too many words which have been picked up at different times, by people from different dialects, and had either the spelling or the pronunciation change in different ways.
I seem to recall that all languages have rules, even English. A rule doesn’t mean there can’t be exceptions. French is perhaps the second most irregular popular language, and after awhile, you can make a pretty good guess at how a new word is pronounced. English is a a frankenlanguage hybrid between Anglo-Saxon and Norman French, so got strange aspects of each.
I have a name that is fairly common in the US, but not phonetic in English. I still get people who somehow can’t pronounce it, or can but make comments about how it “looks weird.” The language it comes from may look hard to pronounce to us, but is rather regular.
It’s an imitation of Shaw.
Among other weaknesses, I’ve never quite understood the P here. Do Brits say “hiccup” and write “hiccough”? Because I always understood them to be different words, and I’ve heard people pronounce the latter to rhyme with the obviously-related “cough”–that is, with an “f” sound.
I am extremely descriptivist – ‘broo shetta’ is a totally acceptable pronunciation, especially in English. I would never think less of someone for using it.
What I can’t bear is when people ‘correct’ me when I pronounce it the ‘original’ way. That is annoying: they combine an ignorance of language (since presumably they think that they are correcting me about the Standard Italian) and an ignorance of linguistics (since they are prescriptivist in this correct-a-stranger way).
pdts
i wouldn’t correct you, but i’d think you were really pretentious. 
How about an American who grew up in an immigrant family where Italian was spoken at home, who studied Italian from an early age as her main second language and could speak, read, and write it with fluency, and, upon reading a new Italian word, could not help hearing it in her mind with the standard Italian pronunciation? And was honestly incapable of predicting the random ways Americans will mangle Italian words and names? Do you call that “pretentious” too?
To me, that word would literally mean pretending to be someone you aren’t. But that isn’t how I hear it used these days, since it became so popular. It’s more often used to mean ‘unable or unwilling to dumb oneself down to the most dumbed-down level of all’ or ‘daring to let on that one has more learning than a high-school dropout’ (the nerve!). The funny thing is, the people who fling the word around here at SDMB so carelessly are certainly better educated than that.
So people who misuse the word in this way are ironically behaving very pretentiously. 
Since there are no English words deriving from Anglo-Saxon that use the letter combination <sch>, that criterion is eliminated as a basis for evaluating its pronunciation. There are, of course, words deriving from Greek such as scheme, schizoid, or scholar; from Dutch, such as schipperke; and even, wonder of wonders, from Italian, like scherzo. Then there’s schooner, of unknown etymology (it sounds Dutch to me). Many of them words that even high-school dropouts know how to pronounce. So it’s hardly a slam-dunk to assume that an Italian word must be pronounced according to German orthography. Why German? I really have to wonder. Without bringing loaded, overused words like “pretentious” into it.
BTW, we got the word schizophrenia from German (it was coined in German as Schizophrenie by the Swiss psychiatrist Eugen Bleuler in 1910). Go figure…
I’ve heard it’s because so many Italians-Americans have specifically Sicilian routes - e.g. ‘pasta fazool’ instead of ‘fagioli’.
We have to consider the fact that 17.1% of the American population has German ancestry. That’s more than have English ancestry. More than have Irish, and far more than have Italian. German is the number one ethnicity in the US, in fact. So there’s that, with all of the German names like Schwartz and Schneider. Not to mention Anheuser-Busch. “Sch” just looks like [∫] to most Americans, despite the counter-examples you provide.
All of them, as far as I know. 
Unfortunately they also all have native spelling.
Indeed. In fact, an Italian teacher once explained to me that the “h” in spelling serves to indicate that it isn’t “bru-shett-a”, but “brusketta”. The “i” serves an analogous purpose before back vowels (a, o, and u). This same teacher told us that the “i” in this case also is just another orthographical symbol and not pronounced. So the Ciu-Ciu wine designation is pronounced much like “choo-choo”, and not "chi-u - “chi-u”. Giovanni would be pronounced more like Joe-vonny, and not Gee-oh-vonnie like most English speakers pronounce it. Of course I wouldn’t be surprised if this varies with different dialects.