I write what I want to write. I accepted long ago that, baring some tremendous streak of luck, I'll never make enough writing to make it worth anything. (i.e. if I'm super lucky, I'll earn approximately what I would have at a minimum wage job) As such, I do it for myself. Markets be damned.

That said, if I write something that feels appropriate for a market only with a few tweaks, I tweak it. I see nothing inherently wrong with say, padding a story out a bit, or cutting a bit, or whatever to make word count. Or changing the theme to meet an anthology, or whatever. TBH, I've now written two stories specifically for anthologies just to see if I could. (Hah, we'll see if either is accepted.) I see nothing wrong with writing to a market - it's actually a fun challenge. So would I write something I thought was garbage for a market? Would I write about a theme that bored me? Hell no. But I'm totally willing to fiddle with an idea I've been given by someone else.

I honestly think that book publishers are probably more open than most others, because it's cheaper to publish a book than, say, make a movie or even record an album. This gives a certain degree of freedom in betting on unusual ideas/new authors/whatever. That said, I haven't tried to sell a screen play or a CD, so who knows?