they don't, because it's against instagram's TOS to sell shoutouts, you won't find this anywhere in the fiverr TOS to the letter though, but as a general rule, if something is not allowed on a third party site, it won't be allowed on fiverr either
http://support.fiverr.com/hc/en-us/articles/201739743-General-Terms
"Sellers advertising their Gig online must comply with laws and terms of service of the advertising platform or relevant website used to advertise. Failing to do so may result in removal of the Gig and may lead to the suspension of seller's account.
By offering a service, the seller undertakes that he has sufficient permissions, rights and/or licenses to provide, sell or resell the service that they offer on Fiverr."
you'll still find gigs like that all the time, they somehow slipped through the reviewing process and haven't been removed automatically or haven't been reported yet
