Why criminalise dialogue with terrorists?

This article by Andy Carl and Sophie Haspeslagh, both of Conciliation Resources, discusses the adverse effects of counter-terrorism  legislation on peacebuilding efforts.

When the US, Britain and the European Union proscribe terrorist organisations they effectively blacklist them.  This criminalises many forms of contact with them and has the effect of directly challenging a mediators' ability to engage with them, even if their intention is to support negotiated settlements.  Also, hardliners within armed factions can become more entrenched as a result of being blacklisted, with civilians left to pay the price.  

The article argues that the laws and their exemptions need urgent review and clarification in light of these unintended consequences.