Why do PDL's only take Western Union payments?

Probably because most of them do not want anyone to have a real, physical address.
They can remain more anonymous and difficult to track this way.

because with exception of the members here who work in collections or recoveries.alot are JDB'S,rent-a-lwayers,or illegal debts to begin with.they don't want you have a physical address to send something other than a mnoey order.know what i mean?if they are going to threaten everything under the sun.they won't give an address.they don't want to give out the address because they know that people will send the proper validation request or better yet a summons.