They wouldn’t. The only "free" accounts are either freshly created empty shells or stolen accounts that will be locked within hours.
If your goal is to remove the "Limited User" status on a free account (which blocks adding friends or using the market), you don't need a shady third-party site.
: Websites claiming to give away free premium accounts often require you to download "account checkers" or "generators." These are frequently disguised malware designed to steal your own login credentials.