It's the behavior when using the default `Encoding.UTF8` static. You have to create your own instance as `new UTF8Encoding(false)` if you don't want a BOM.
I actually did not know that File.WriteAllText/new StreamWriter defaulted to UTF-8 without BOM if no encoding was specified. I always passed in an encoding to those functions, and "Encoding.UTF8" has a BOM by default. Without specifying any encoding, I just assumed it would pick your system locale, because all the default String <-> Number conversion functions will indeed do that.
There are some coding standards for C# that mandate passing in the maximum number of parameters to a function, and never allow you to use the default parameter to be used. Sometimes this is a big win (prevents all that Current Culture nonsense when converting between numbers and strings, you need Invariant Culture almost all the time), and other times introduces bugs (Using the wrong value when creating Message Boxes to put them on the logon desktop instead of the user's screen).
Enforcing an overload of the highest arity of arguments sounds like a really terrible rule to have.
Culture-sensitivity is strictly different to locale as it does not act like a C locale (unsound) but simply follows delimiters/dates/currency/etc. format for parsing and formatting.
It is also in many places considered to be undesirable as it introduces environment-dependent behavior where it is not expected hence the analyzer will either suggest you to specify invariant culture or alternatively you can specify that in the project through InvariantGlobalization prop (to avoid CultureInfo.InvariantCulture spam). This is still orthogonal to text encoding however.
