The annoying thing is the deal probably isn't even needed. The DUP are long standing allies of the Conservative party and would hate Corbyn as PM. Not only would they prefer the Tories in power anyway, they wouldn't dare vote against them in confidence votes and risk Corbyn. The Tories could have refused any deal, spared themselves association with such reactionary extremists and spared the embarrassment of publicly abandoning even more of their manifesto, just throwing a few bones later on to maintain friendship.