Why do NEB adapters have non-complementary sequence?
The original point of the non-complementary sequence is to allow formation of a Y-adapter after USER treatment, since that's what's needed for the Illumina chemistry to work. In reality the bottom strand may not be there in step 3, since USER has already digested one strand (creating the hairpin in step 2) and size selection has been performed. Regardless, it'll get removed once cluster formation is performed.