http://www.immihelp.com/forum/showthread.php/109726-Outside-US-with-expired-greencards-please-help
That's a very good question, and I'll answer it. But before we get to that, let's make something perfectly clear - your parent's aren't, and haven't been for a long time, lawful permanent residents. Let's have that crystal clear in your mind. Please spend some time going over this link - http://www.immihelp.com/greencard/retain-greencard.html - specially the section on "Maintaining Permanent Residence".
So with that clear, let's talk about "but then how did the USCIS send them a letter for fingerprints?". That is yet another example of the government's right hand being completely oblivious to what the left hand does. The truth is that the USCIS doesn't track your coming and goings - they actually rely on CBP to do so for them. The CBP POE officer is the one that, every time an LPR reenters the country, has all the information available about when that person left and came back. That's why (and please Google for it) you will find many, many stories like "I came back after 9 months, got a warning from CBP" or "I been leaving and coming back once a year for the last three years - last time my greencard was cancelled" and such. So - USCIS is blissfully ignorant, and may even assume your parents haven't left the US since 2006. So up to that point, you're (so to speak) OK - IF your parents were in the USA, they would be LPRs that "forgot" to renew their greencard - no big deal. Apply for renewal (as they did), get a new one, move on.
But they're outside the USA - and to attend the FP interview they have to come back to the US. And again, let's be clear here - even if they were able to pull that one off, they would be doing something illegal - again, they can't claim to be LPRs when they didn't fulfill the conditions to keep being so.
Anyhow - they need to board a plane to the US, arrive to POE, go thru CBP. And that's your problem, as you stated. No airline will let them board with an expired green card - solution would be to apply at the nearest consulate for a boarding foil - which probably wouldn't be granted, as the consular officer would ask "why didn't you renew your GC before? How long have you been outside the US?". And that option disappeared when they applied for SB-1 - which was denied, as you state. So the consulate probably DID check the CBP records - to find out your parents have been outside the US for almost seven years - hence, no more an LPR.
They COULD try to apply for a B1/B2 visa - in order to travel to the USA - but they will need to surrender their GC. Again, because an LPR doesn't need a visa to travel to the USA (they're assumed to RESIDE in the US). And your parents are already in the system, as they applied, and were denied, an SB-1 visa. So - I don't see a B1/B2 visa in their future, even if they surrendered their GC - they already lied to the consulate.
And even if they did NOT lie on their SB-1 application - again, were they to be granted a B1/B2, they would need to surrender their GC, *relinquishing their status as LPR* - and hence, even if they were to arrive and be admitted to the USA, and show up for the FP appointment - the problem here is that they surrendered their GC - and that information will make it to the USCIS.
Even in the very unlikely scenario where they surrender their GC, get a B1/B2, arrive to the USA, are admitted, attend the FP appointment, are mailed a new GC - they broke the law, as they applied for a benefit (the GC) for which they don't qualify anymore. So now they are criminals - and subject to immediate deportation as soon as the issue is discovered.
Now - is the situation clear to you ?
