I'm not a Vercel fan, but the whole pitch of PaaS is you get more than just a provisioned server for your application. The 20$/month/dev is vercel's own concept of what that Dev Experience costs with a profit margin + average usage fees paid to AWS baked-in. They might leave that average low, but they assume you're here because you like vercel's tooling, not that you're price shopping for $/BitsTx'd. AWS will always win in that, because vercel is also AWS with some dipping mustards they really want you to lock into.

The hobby plan is a loss leader to get developers into the vercel tooling. If you go over the free tier's bandwidth limit, you've exceeded what vercel believes that developer goodwill is worth for a single account. If they allowed you to pay for extra bandwidth on your free plan, it would make vercel look like a crap cloud platform, because all you're doing at that point is paying a premium for AWS, and a kneecapped version of their developer tooling. They really want you to pay the 20$/month/dev and experience everything in vercel's platform because that's their only product. Honestly... no fault to them on that.

Maybe they'd gain some developer positivity about letting you dig your account out of the "exceeding the hobby limits" hole that's easy to fall into, but the AWS cost for them is already spent, and that was all the budget for appeasing you. You'll have to pay them to pay AWS anyway, so they draw a hard line at that point and demand you also pay to use the vercel tooling, which is the only thing they make. (or, in theory, telling you to go pay AWS yourself if the tooling is unimportant to you.)

They will sell you pay-as-you-go services... but only once you pay their 20$: https://vercel.com/docs/limits#on-demand-resources-for-pro

Over all, I hate it. But I don't think there's anything too hidden about it, or at least no more than any other PaaS provider.