Howdy Chris,
Events Calendar Pro actually already is responsive, if your theme supports it. As of version 3 even Photo and Map views are responsive, and the Events bar. Of course the others views are as well. Some responsive sites have chosen to disable month view, simply because trying to display 7 columns (one for each day of the week) side by side in 320px is not always feasible without horizontal scrolling. But, technically the view is responsive if your data and theme-CSS support it as well.
As you point out our demo page is not responsive though. This is because the theme author has not yet been tasked with creating an updated version. I apologize for the confusion this creates. However, if you use our stock plugin with a responsive theme, like the default WP one, you will indeed find it to be very responsive.
Does that answer your question? Thanks for posting!
– Brook