Running .sql script file from sqlplus succeeds, but passing the sql string as standard input of sqlplus fails with SQL> SP2-0734 in powershell

Question

I am trying to execute an sql script with powershell and sqlplus but I am getting inconsistent behavior.

I have a file query.sql. It has one line as :

SELECT * FROM mytable;

When I execute it with

PS C:\MyScripts> sqlplus {myUser}@{myConnection} "@query.sql"

The command returns the results correctly. But when I try

PS C:\MyScripts> "SELECT * FROM mytable;" | sqlplus {myUser}@{myConnection}

the sqlplus returns and error:

SQL> SP2-0734: unknown command beginning "SELECT ..." - rest of line ignored.

How can this happen and what does it mean?

I expect that the same command can be executed with both of the mentioned methods - standard input from PowerShell string and input from a .sql file.