There is a discount of sorts, just not limited to Sketch 2 owners (and as Casimir points out below, if you bought Sketch 2 after March 1st, you get it for free):

For one week only, until April 21st for customers old and new, Sketch 3 will be available at an introductory price of just $49.99! After the 21st, Sketch will return its normal price of $79.99.

The only difference being that people who buy it this week will get it for the discount, rather than just Sketch 2 owners. I suppose trying to offer upgrade discounts is a bit of a headache when selling through the Mac App Store.

I'm happy with the discounted price, and the fact that more people can take advantage of the discount is fine by me.

It's a shame it's for a limited time only, but buying Sketch 3 is a no-brainer for me, so I still see the discount as a loyalty discount just like an upgrade discount is (albeit with a time limit).

http://bohemiancoding.tumblr.com/

"If you purchased Sketch 2 on the Mac App Store after March 1st, 2014, you will receive Sketch 3 for free."