There are a number of gaping holes in our immigration system. One is that their is no non-Kafkaesque solution for foreign-citizen family members of globally mobile American citizens. This is particularly ironic precisely because such mobile citizens are much more likely than average to have foreign family members in the first place.
When it comes to family members immigrating to the US, our system is permanent residency (a green card), or nothing. A would-be resident applies for a visa based on a family status such as marriage, and after receiving it generally has six months to arrive in the US and another ninety days to apply for a green card once he or she arrives. Assuming everything is in order, they get a green card a few months later, and everything is great - unless they ever want to live outside the US. In this case, USCIS throws a hissy.
If you want to live outside the US while holding a green card, you need to obtain a special travel document before you leave, and periodically come back to the US in a pair of closely-spaced trips or one extended trip in order to replace your travel permit if it is about to expire. USCIS also refuses to grant serial travel permits. While there is no fixed standard, spending more than half one's time outside the US as a permanent resident is likely impossible. If a permanent resident fails to maintain their travel permits, USCIS will strip their residency and green card upon entry to the US. This ends up creating tremendous headaches for the foreign family members of US citizens, forcing them to spend thousands of dollars in filing and legal fees, not to mention the cost and time of repeated trips back to the US from abroad at USCIS's whim.
Is there a solution to this costly, pointless issue? In fact, there are at least two, as exemplified by Canada and Japan.
The Canadian system simply allows its permanent residents to live abroad if they are living with their Canadian citizen family member, or if their Canadian employer moved them abroad. From the Canadian perspective, the maintenance of one's Canadian work or family ties is sufficient to protect one's residency, even if abroad for years at a time.
Japan is also very relaxed about its "permanent residents" living abroad. However, it has an additional feature - long-term family visas. In Japan, one does not jump straight to permanent residency. Instead, family members, like workers and students, start with 1-5 year visas that provide residency and work rights. These generally are indefinitely renewable unless there is a cause to deny them, and in many cases, Japanese foreign residents never bother to obtain their "permanent" residency and are content to remain in Japan on a series of family visas. If such people want to leave Japan for a few years, it is no big deal at all. Just leave. If your visa expires while abroad, apply for a new one before you come back. Or even after you come back, as unlike the US, you can apply for visas in Japan while in the country as a tourist. Given that Japan's visa applications require half the documentation, cost five times less, and are adjudicated in less than half the time than the US, it is perfectly possible to land as a tourist and get your visa after the fact.
It's a globally mobile modern world, yet our immigration policy is still based on archaic rules set in an area where coming to or departing from the US was a long, expensive one-in-a-lifetime event. It's time we moved our immigration policy out of the stone age and made it simple for our permanent residents to move in and out of the country.