Sed replace string if first letter is uppercase using sed

Sed replace string if first letter is uppercase using sed

I try to write sed answer to this question Edit a file using sed/awk using:
sed -e 's/^[A-Z]/$:$&/' file.txt
but the result is:
wednesday
$:$Weekday
$:$thursday
$:$Weekday
$:$friday
$:$Weekday
$:$saturday
$:$MaybeNot
$:$sunday
$:$MaybeNot
$:$monday
$:$Weekday
$:$tuesday
$:$Weekday
Why it replace if first character is lower case?