How to get psql to prompt for a password with the superuser postgres?
why when I log as a normal user it asks for a password and doesn't when i log as superuser?
  
  it just doesn't seem safe at all. 


The small but important detail that you're missing is that nobody but you (or your DBA Team) should be allowed anywhere near the database server and absolutely should not have the postgres account credentials.  Without these, "logging in as postgres, with or without a password" simply won't be a option for them. 

Remember: As the DBA, you'll have to cleanup the "mess" that other people make, so always keep the biggest and best tools for yourself!