Why would it matter who is signing in as long as whoever does the work follows the same agreement standards? Sounds an awful lot like controlling what will be done and how it will be done, which is the definition of an employee.
I guess I can't imagine a situation where Bill is contracted at a temp agency, turns around and hires Bob, and when Bob shows up with Bill's badge and is refused entry, and Bill somehow wins a lawsuit on the theory that Bill must be allowed to subcontract and anyone he subcontracts to must be allowed on-site to perform the work, no matter what.
Like, not giving out your password (or badge, or key, or whatever) to someone else is a pretty core job responsibility!
If they are contracted at a temp agency, then they are taking on the risks Bill would be an employee of the temp agency... Employee misclassification is frequently abused so don't get me wrong, I think it happens a lot, but in many cases people are misclassified.
