Offering relocation assistance and hiring people who don't live in the area are two different concerns.
Typically, job postings that offer relocation are offering relocation assistance. Often, this is some kind of compensation to cover moving expenses, finding a place to live in the new job location, and other things associated with getting from your current location to the location of the job. The kind of assistance varies by company, and could range from most or all expenses paid, to a sliding scale based on relocation distance, to a flat rate to offset some of your costs.
Accepting applicants from outside the country is a different matter and depends on the company. If you aren't a US citizen, there are companies that can not hire you due to the nature of the work. Others can, but choose not to sponsor the applicant for the required documentation for whatever reason. Typically, if a company requires US citizenship (or some other legal status), that's noted in the job posting. Companies that can may also note that they will sponsor the applicant for the required documents in the job posting. If neither are mentioned, it may be wise to contact the recruiter or HR department before getting too involved in the process.