What is their rationale? I saw one rant on Reddit where the dev was angry that OUYA forced him to give away his work for free and people would just play a demo then never pay. Are they all like that? As I understood, the technical implementation is fairly straightforward, right?

The core problem is that it appears player-friendly, but in practice it is not, because it wounds developers. If you're an indie with limited resources, being forced to build a demo means you end up having to ship a bad demo (actually reducing your sales in many cases) or not ship on the platform at all.

In an ideal world, 'all games have free trials' is good. The realities of game development mean that even for games where it's possible to build an excellent trial, it may not happen. Budget cuts happen, resources you thought you had go away, your schedule slips or deadlines draw closer - in those scenarios, it would be unreasonable to make the core game suffer in order to have a demo ready by the deadline. Most developers would instead focus on the most important part - the core game, the thing people are paying for.

There are also games for which it is near-impossible to build a free trial. A recent example is The Stanley Parable; the nature of the experience made it impossible to have a 'free trial' version of the game, so they had to instead build an entirely separate game that they then gave away for free, and tried to use to convince people to buy the actual game.

That solution worked great for them, but it took time and resources. I wouldn't be able to bring myself to tell every developer they had to do that just to be on my store.

Note that demos and trials are great when they work: They help convert slight interest into sales. However, many successful games sell entirely on the strength of good reviews, word of mouth, and press coverage. This is part of why sometimes the wisest choice is not to have a demo.