Though most open source licensed projects allow anyone to come along and access the source, the strict interpretation of the GPL for instance, is that those rights are only extended to customers/users of the software. Those customers are perfectly within their rights to distribute it openly in turn - but as I understand it, neither the the copywrite holder(s) of the source nor the providers transmitting a GPL project to an end user are obligated to provide a copy to any person who asks. Only that particular user who was provided the binary.