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Why does redirection, using sudo give me an error for the following commands? 

$ sudo printf "foo" >/etc/file
bash: /etc/file: Permission denied

$ sudo printf "foo" ~/file; cat ~file >/etc/file 
bash: /etc/file: Permission denied

...yet I have no such problem when I use an editor, or cp.
I don't think I've ever tried this before, so I don't know if there is something haywire on my system, or if this is normal. It seems a bit restrictive to be normal, but(?) maybe it is intended to be restrictive... (using Ubuntu)

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It is normal. The file after the > is not open by the process running under sudo, but by the shell, which isn't. Try this instead:

printf "foo" | sudo tee /etc/file
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    Got it, thanks.. so this works too, sudo sh -c 'printf "foo" >/etc/file'
    – Peter.O
    Aug 30, 2011 at 3:37
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    Also, as mentioned in the Q/A linked to by Gilles comment, tee -a caters for appending to a file...
    – Peter.O
    Aug 30, 2011 at 12:15

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