Why does Oracle require TO_NCHAR when binding SQL_C_WCHAR text via ODBC

SELECT (CASE profile WHEN ? THEN 1 ELSE 2 END) AS profile_order
FROM engine_properties;

SELECT (CASE profile WHEN          '_default'  THEN 1 ELSE 2 END) AS profile_order
FROM engine_properties;

SELECT (CASE profile WHEN TO_NCHAR('_default') THEN 1 ELSE 2 END) AS profile_order
FROM engine_properties;

SQLPrepare(SELECT (CASE profile WHEN ? THEN 1 ELSE 2 END) AS profile_order
    FROM engine_properties;) = SQL_SUCCESS
SQLBindParameter(hstmt, 1, SQL_PARAM_INPUT, SQL_C_WCHAR,
    SQL_VARCHAR, 32, 0, "_default", 18, 16) = SQL_SUCCESS
SQLExecute() = SQL_ERROR
SQLGetDiagRec(1) = SQL_SUCCESS
[SQLSTATE: HY000, NATIVE: 12704, MESSAGE: [Oracle][ODBC]
    [Ora]ORA-12704: character set mismatch]
SQLGetDiagRec(2) = SQL_NO_DATA

SQLPrepare(SELECT (CASE profile WHEN ? THEN 1 ELSE 2 END) AS profile_order
    FROM engine_properties;) = SQL_SUCCESS
SQLBindParameter(hstmt, 1, SQL_PARAM_INPUT, SQL_C_WCHAR,
    SQL_VARCHAR, 32, 0, "_default", 18, 16) = SQL_SUCCESS
SQLExecute() = SQL_ERROR
SQLGetDiagRec(1) = SQL_SUCCESS
[SQLSTATE: HY000, NATIVE: 12704, MESSAGE: [Oracle][ODBC]
    [Ora]ORA-12704: character set mismatch]
SQLGetDiagRec(2) = SQL_NO_DATA

SQLPrepare(SELECT (CASE profile WHEN TO_NCHAR(?) THEN 1 ELSE 2 END) AS profile_order
    FROM engine_properties;) = SQL_SUCCESS
SQLBindParameter(hstmt, 1, SQL_PARAM_INPUT, SQL_C_WCHAR,
    SQL_VARCHAR, 32, 0, "_default", 18, 16) = SQL_SUCCESS
SQLExecute() = SQL_SUCCESS
SQLNumResultCols() = SQL_SUCCESS (count = 1)
SQLFetch() = SQL_SUCCESS