Nothing complicated. Simply that it’s easier to not have a check for None (using .get()) over doing so. It’s a minor point, certainly.
Re-reading that FAQ entry, I am uncomfortable with it, but (I suspect unfortunately, from your perspective) mainly because I think it’s simply making excuses. The reality is simply that as PEP author I think those fields should be mandatory. And my reading of the discussion here is that consensus is in favour of a version bump, but (with the exception of you) people are fairly indifferent about whether fields in general should be mandatory. So let’s look on a case by case basis.
There’s no question in my mind about versions. Being able to get a list of project versions is essential for many types of enquiry, and releases with no files are possible (at least for PyPI) so only the server can provide that information reliably. Servers that have no better means of doing so, can extract the version list from the file list (which they are serving anyway).
For the file size information, every file has a size, so there’s no sensible reason to make the field optional. And once again, some clients have good reason to need it and the server has the data so it should publish it.
Regarding the upload time, certainly there might be servers that don’t record this (or files for which that data isn’t available) so there is an argument for allowing it to not be returned. The PEP currently says the field is required but can be None. I’m OK with changing that to say the field is optional, but if present must be a valid date (that actually follows the lead of fields like gpg-sig and yanked). If no-one says they prefer one approach over the other (or there’s an argument, which I doubt!), I’ll make an executive decision. I’m not going to over-constrain here by saying that if any file has an upload date, every file must, or anything like that. Servers should (and I hope, will) provide what they have, so demanding what they can’t provide is pointless.
The FAQ section, I’ll delete. The PEP shouldn’t be making excuses for itself.