big hosting companies? well it's basic stuff ..
you log in onto your servers through ftp user/pass ..
user logged in, uploads a file .. automatically it's owner is user and group the group the user is in .. after that, if you want that file to be writable through the webserver, you need to change it's file permissions MANUALLY ..
If i know you are hosting a server with everything world writable, i'd like to hire a hosting package with you and have some fun!
.. if you know what i mean ... it's just a "not done" thing.
if you want a file world write then do chmod o+w <file> and you should be fine