Open Source is, by and large, intended by the creator to donate their ideas to benefit humanity as a whole; many people feel that using their thing to help strengthen an (ethically questionable) monopoly is acting against that core goal.
If AWS/Azure/GCP/et al. ran a cloud version of X and the main company supporting the open source project was a going concern, I doubt many would have a problem with the entire scheme.
However, in reality, every enterprise support dollar that goes through a third party cloud-managed offering is one that doesn't go to the first party.
In which case, what dollars are left to pay the independent company that creates and supports the software?
Granted, there are a lot of nuances to the above, but I think it's generally fair to say that third-party cloud companies are making more off managed open source offerings than they're paying to contribute to them.
The only losers are people who are engaging with the Open Source community in bad faith, viewing it as something to steal from, rather than participate in.
I don't think anything else open source I've done has been widely deployed, but if I save a bit of someone's time because they can use something I did, or save some users' cpu and bandwidth, it doesn't matter to me if that's a user of a free service or a propriatary one, I still helped their user.