It's actually fairly hard to get money onto those without paying a fee (if you don't have a bank account). Most of the obvious ways to get money onto them involve "Visa ReadyLink", and that comes with a service fee from $2 to $5. If I recall correctly, some even have monthly fees if they get below a certain level.
Amazon gift cards can be purchased without a fee most of the time, and with less friction in the store then actually dealing with the service desk in the store that does ReadyLink.
