What permits companies like Elance to employ freelancers from foreign countries without a visa requirement? Normally one needs a special type of visa to work for a company in USA.
What permits companies like Elance to employ freelancers from foreign countries without a visa requirement?
The TL;DR; answer is: Because (in the example you ask) the freelancer is not working IN the USA - they are supplying a service or delivering goods TO, or perhaps working FOR, a company based in the USA.
Visas permit the movement of people, and restrict what they do in a country. You do not need a visa to work in your own country... just as you do not need a visa to post to any StackExchange website
What permits people to hire the services of contractors oversea without a visa?
Basically for the same reasons why they don't pay taxes or vote in other countries. It's also something that's very hard to control with internet now and most countries simply freely allow this service trade with selected "friendly" countries.
There's no real visa, but there's still a common set of rules they share and that act as a "worldwide visa" in a way; with the "non-friendly" countries, that freedom is not given so easily as the governments will monitor transactions and communication (or make all this impossible.) For example, I doubt there's any North Korean freelancers on Elance. The agreement of friendly countries is something like "manage your people, I'll manage mines and let's keep in touch." That's the easiest and cheapest way to manage international trade when there's no central power.
A visa is like a permission and also a guarantee that the person to who it's granted will get some protection but will also need to follow the laws and regulations of the country. It's like a temporary contract.
Technically, the reason why they don't need a real visa when doing freelance oversea is because the freelancer doesn't gain/require any benefits from the country of the buyer and doesn't have any extra right/protection in it either; the only protection of a freelancer in this case is the laws and regulations the buyers must submit to in his/her own country. The freelancer doesn't cost anything to the buyer's country if he follows the basic international rules (eg. no electronic fraud or money laundering or illegal trade.)
In theory, the freelancer doesn't have any obligation towards this country either, besides the special treaties and accords his/her own country has with the country his/her employer is located.
I'm not a lawyer but I guess that's the simplest way to explain why it's legal and doesn't require a real visa.