Craigslist charging employers to post job listings doesn't seem like it would have much of an impact on the consumer market. I'd guess that the business-to-business expense in this instance probably isn't significant enough for it to be passed on to the (employer's) customers or impact their experience.
Restaurants probably don't take as large of a chunk of their revenue or have as many restrictions placed upon them by Grubhub as Spotify does by Apple.
Amazon, on the other hand, is probably the closest comparison. However, I suspect Amazon's services to their sellers involve greater capital and operating expenses than the services Apple provides to their sellers. Both companies need to spend money on tech infrastructure, development, and support. In addition to this, Apple needs to employ reviewers to act as the gatekeepers of the App Store and possibly buy hardware for those reviewers to use. In contrast, Amazon needs to build/purchase/rent warehouses, employ warehouse workers, and purchase warehouse equipment (forklifts, etc.).
Amazon also doesn't take a cut of, for example, Microsoft's subscription fees if someone orders an Xbox and subscribes to Xbox Live. This might be different for Amazon's app store for their Fire OS devices, but I'm guessing you were referring to their general marketplace.
To be fair, I probably don't know enough about any of these markets for my opinions to hold much weight, but I'm not sure if the examples you gave really line up with the situation between Apple and Spotify. But I definitely agree that it's important for people to try to remember to stay unbiased with these things.
But I would just caution people to consider whether your stance is truly based on what you think should apply generally, or just because you like the person/party in the specific case. (applies to a lot of things in life)
The power of platform providers versus app providers ebbs and flows back and forth with new technology, depending on who has the most market / customer pull. And you should try to deconfuse your principles about who should be able to charge what, from your liking of the provider -- because that can change in an instant.
Craigslist charges employers a significant fee to post their job listings. Employers have far less sway than Spotify does in that platform dispute. I'm guessing you (audience) like Craigslist, and support them in what they charge? Why?
Amazon charges companies to sell on their platform. You probably don't complain a lot about that, and that's probably just as restrictive as Apple+Spotify for some sellers. Why? Is it because you like Amazon's services?
Or you maybe like Grubhub, and don't care that it takes a huge cut from restaurant owners. (a cut that is more painful than what Spotify charges per month, to the restaurant on every single order you make)
I don't choose sides either way. What is "reasonable" is what the parties had the clout and market pull to agree to, and what the law says is acceptable in form.
Separate your principles from your preferences.