The UK, Ireland and Denmark (around the time of the Treaty of Amsterdam and the creation of Schengen) negotiated opt outs from EU law in the areas of freedom, security and justice. As such, any provision in such areas created by an EU directive is optional for them.
Based on the information available (see page 19) it appears Denmark has opted out of every relevant treaty provision, whereas the UK and Ireland have chosen to implement many of the provisions of the CEAS on their own. No other member state has such an opt out, and it would be up to any new applicant state to negotiate one if it wanted one.
The CEAS attempts to coordinate and unify the EU policy on asylum seekers. This is both necessary in a system with open internal borders, and imposes significant theoretical responsibilities on countries, above those in general international law, since the relevant international law is extremely ambiguous.