Because s/he already has a visa! The problem usually is that permanent residents have a long waiting period before they can obtain visas for their spouses and children. They will also normally be denied visitors' visas by consular posts as they are unlikely to be able to prove that they do not have immigrant intent. However, if you are already in the country with a valid visa, it is simply a matter of maintaining status, one way or other, till your spouse is able to obtain citizenship. Even if a person in that situation is unable to maintain his/her status, it is very remote that they will be deported or they will be denied immigration benefits when the turn comes. The only constraint is that, if s/he fails to maintain proper status, s/he may not be able to travel outside the country and come back freely till she is able to obtain her green card.