Someone wants to make an easy profit off the coat-tails of Apple's success
Why can't complementary products exist? You wouldn't consider a train timetable app parasitic. In fact, my local train network encourages such apps so that they don't have to do it themselves. Businesses can build co-operating products.
In fact, when the Pebble first came out its notification handling was better on iOS than Android because Apple specifically made it easy. Pebble, Fitbit, and Garmin paved the way for smart-watches. Once the hard work was done, then Apple produced their own. And what a coincidence, everything gets locked down for third-party watches.
Nobody is complaining that they don't make things easy. They're complaining that they make it impossible because they compete with Apple.
If one or more companies has the ability to lower the price of a good or service, that's good for consumers.
If one or more companies has the ability to raise the price of a good or service, that's bad for consumers.
The complaint here is that Valve, Apple, and Google have used their position to increase the cost to consumers while reducing the profitability of game makers.
I'd be less concerned about bad actors being able to install the software. I would assume that the phones would still verify the installed packages.
But this probably means that if you are concerned about some threat, you can't just assume that a switched-off phone is safe.
You know, Callahan's is a peaceable bar, but if you ask that dog what his favorite formatter is, and he says "roff! roff!", well, I'll just have to...