[plt-scheme] regexp-match on input port returning bytes

Just out of curiosity, in PLT 4.1.2, why does "regexp-match" on an input 
port return bytes rather than strings, when using a string regexp (not a 
bytes regexp)?

 > (regexp-match #rx"^x" "x")
("x")
 > (regexp-match #rx"^x" (open-input-string "x"))
(#"x")
 > (regexp-match #rx#"^x" (open-input-string "x"))
(#"x")

I would've expected the second example to yield a list of strings rather 
than a list of bytes.

Thanks,
Neil