In 36:10-11, it says the daughters of Tzlafchad did as Hashem had commanded Moshe and they married their cousins.
From the fact that it says this only after the representatives of Menashe petitioned Moshe on the subject, and from how it says it here that they did as Hashem commanded and married their cousins, I would suggest that we can deduce that the girls originally intended to marry out of the tribe and not marry their cousins. Perhaps that is why the reps of Menashe felt the urgency in suddenly petitioning Moshe - they were about to lose that portion.
The passuk here testifies that they did as Hashem commanded. They did not come forward with counter-arguments. they could have argued that they should not be limited, they could have said the case was already decided with no such condition, etc. They could have put forth a number of arguments. but they did not. They did as Hashem commanded, and married their cousins.
This is a testimony to their great fortitiude, and to their integrity in the sense that they were all along really just trying to do the right thing, and not personally profit from the loophole. They changed their plans, they cancelled their plans with whomever they were considering marriage, they made no peeps or complaints about it. They simply did what Hashem said to do.
We should all be so honest and deal with such integrity in our dealings.