It is the general consensus that chemically treated hair absorbs water more easily than virgin hair. This difference is noticed when trying to wet hair in the shower prior to the shampoo application. It takes longer for virgin hair to get completely soaked.
The effect can be attributed to one important fatty acid on the hair surface: 18-MEA (which is short for 18-methyl eicosanoic acid)
18-MEA is bound to the thin outer layer of the hair cuticles, and is often referred to as the external protective F-layer (lipid layer) of hair. The fatty acid attaches itself to the cysteine of the protein layer. This layer has been known to provide pronounced hydrophobicity to hair. In other words, it does not like water, and tends to repel it. [More here about the hair structure.]
This characteristic allows the lipid layer to control how much water gets absorbed into hair. Too much absorption results in extensive swelling/stretching. With that being said, it also controls how much water leaves the hair. As long as this hydrophobic layer is present, less moisture is lost from the hair.
Other than being hydrophobic, 18-MEA helps keep the hair shiny (like oiled), pliable, and manageable. It also protects the cuticles beneath from damaging effects during grooming or other.
After chemical treatments
During chemical treatments the bond between the 18-MEA and the cysteine gets severed. As the 18-MEA layer is lost, the hair is left hydrophilic (loves water) and the cuticles are exposed to further damage.
Compensating for the loss of 18-MEA
Chemically treated hair needs to be conditioned more. Some research [such as this one] have attempted to deposit conditioning molecules that can mimic the behavior of 18-MEA. The goal is to impart some level of hydrophobicity that can prevent moisture loss from hair, control the moisture absorption into hair, protect hair against further frictional damage and make hair manageable.