Dan Edwards, writing for Medium:

Recently Tweetbot 4 was released as a cross-platform update that’ll work on iPad & iPhone. Right now (at 50% off), it’s a $4.99/£3.99 app. Regardless of whether you bought the old Tweetbot recently, or at all.

Some people were pretty angry about this

I can understand people not wanting to pay for a piece of software when there are free alternatives. I can understand people not wanting to pay for an update if they feel the value of the update is just not worth the cost.

What I cannot understand is someone getting upset about a developer charging for an update.

Developers have mouths to feed, bills to pay. If someone builds a wonderful piece of software (Tweetbot definitely fits in this category), perhaps it helps to think of it as you supporting that developer. Paying for their app or update is you helping that developer keep the doors open.