What allows for no selection pressure against non-coding DNA?
A) Here's the thing about evolution - just because something might have an adaptive benefit does not mean that trait will evolve ; this is contingent on the occurrence of the necessary mutations to produce a phenotype in the first place. 

B) There are multiple copies of the transcriptional machinery (RNA pol, Transcription Factors) - numbering in the tens of thousands when you are talking of the core transcriptional machinery. 

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC84381/

The 3d structure of chromatin , as well as the large amounts of the core transcriptional machinery, prevent there from being too much adaptive benefit to losing large amounts of noncoding DNA. 

C) Thirdly, there are suggestions that enhancer promoter contacts, which can greatly facilitate transcription, may be in some part at least determined by sequence motifs in the loops that bring together enhancers and promoters ; so there would be costs associated with losing ncDNA en masse. See www.nature.com/ng/journal/v48/n5/abs/ng.3539.html

D) Indeed, even when one looks at repeat elements derived from endogenous retroviruses, there are those that are of functional significance; Syncitin (which actually codes for a protein) is responsible for trophoblast fusion + placenta formation (See https://www.ncbi.nlm.nih.gov/pubmed/10693809 )  , and there are multiple repeats that themselves serve as enhancers of interferon response genes. See https://www.ncbi.nlm.nih.gov/pubmed/26941318