As far as I know copper membership also allow to send PM without much restriction.
Check the bolded part. The PM restriction is not lifted for copper member.
- Some of the same permissions as Member-rank members, such as reduced signature styling restrictions. (But none of the PM-related restrictions are currently lifted, such as the style limit or per-hour PM limit.)
- Your "you must wait ____ seconds between ___" counter is reduced by 75%. So if you're naturally of Newbie rank, you only have to wait 360-75% = 90 seconds.