The reason is because I read this, which I believe say's that when she applies for permanent status she will have to prove her marriage was not just to get into the US, unless i read it wrong:
"How will a divorce affect the issuance of my conditional green card?
Aliens who obtain their permanent residence based on their
relationship with a U.S. citizen spouse or an alien parent's U.S.
citizen spouse are granted conditional permanent residence if the
qualifying marriage took place within two years prior to the date
permanent residence was conferred. Conditional permanent residence
means that the permanent residence is subject to termination if it is
found that the qualifying marriage was a sham marriage or a marriage
that was entered into only for purposes of obtaining an immigration
benefit. Other than the conditional permanent resident being subject to
having his status terminated, he is afforded the same rights as is any
other permanent resident. Within ninety days before the two-year
anniversary of the permanent residence being granted to the alien, the
alien and spouse must apply to have the condition removed.
When permanent residence is granted conditionally to an alien
spouse, that conditional permanent residence may be terminated within
two years from the date permanent residence was granted if the marriage
has been terminated through divorce. This rule also applies to the
child of the alien spouse who obtains his conditional permanent
residence based on the marital relationship of his parent. In other
words, the general rule is that divorce terminates the conditional
permanent residence. Nevertheless, in this scenario, it is possible for
the alien to obtain a waiver of the termination. A waiver of the
termination is granted to the alien if the alien can show that the
marriage was a union in good faith and if the alien was not at fault
for his failure to file the joint petition to remove the condition.
Generally speaking, if the conditional permanent resident can show that
the marriage was entered into in good faith, it is presumed that he was
not at fault for failing to file a joint petition. Two ways to show
that a marriage was entered into in good faith are proving that the
couple had a child together and producing evidence that the couple
owned property jointly."