If we didn't give new users 1 year free, we risk all our revenue drying up because everyone new customers might choose instead to wait for Construct 3.
Now I appreciate this is only a risk, but it's one we don't want to take. From a business point of view it's a bit of a vulnerable transition and it's important to keep sales going. Between now and end of Construct 3 beta is some months.
Giving first year free for purchases now we consider a semi pre-order for Construct 3, and ensures that new customers won't have reason to hold off their purchases.
We deliberated over this for a long time and felt that 50% discount off first year for existing customers was a good deal. If you don't think it's a good deal, that's fine as always we're not forcing you to take it.