I’d say this is more about security than “greed”. Apple’s giving these other web browser apps free access to their WebKit APIs, it’s not like they’re extracting licensing fees for them… But the reason all browsers on iPadOS use WebKit is ultimately to keep the platform more secure. Other web engines could possibly expose system elements directly to websites, or create other holes in security that malicious websites could exploit. WebKit is more hardened against those types of vulnerabilities. And that doesn’t even factor in the possibility of some of these browser apps maliciously creating loopholes or other such things to collect your info. Who really trusts that Google wouldn’t be baking everything they could into Chrome in order to collect as much of your private data as possible? I certainly don’t. WebKit holds these things in check.I'm aware. It's a significant part of why I'm switching away from my iPad as my primary computer. Apple's greed blinds them and prevents them from making truly spectacular products.
It also has the benefits of ensuring better performance. Other browser engines would likely burn through battery runtime faster, as well as potential other issues. And just look at how much RAM Chrome hoards on other platforms.
If this was just about “greed”, then why isn’t Apple making browser apps pay for the API rather than providing it for free? Because as I said, this doesn’t fit the “greed” motive, but rather seems they genuinely are trying to ensure a greater level of security. What browser engine a browser app uses or doesn’t use doesn’t ultimately change Apple’s revenue at all. So the “greed” claims clearly seem to fall short…
Ultimately, I think it’s pretty clear this is about better system security and not just allowing browser engines to poke all kinds of holes through the security model of the platform. It’s also probably about preserving privacy and maintaining good performance as well. All of these are net positives in my opinion.