So this gets tricky and this is another reason why we can't have nice things. People produce things that look like an official thing, behave largely like an official thing but aren't to spec, sometimes with disastrous consequences like the man electrocuted by a shoddy charger [1].
As we've seen from the Samsung Note debacle a bad battery can destroy the device and even cause fires, which could easily result in death.
Imagine buying a phone battery on Amazon for your phone. The listing looks legit. The battery looks legit. Maybe that seller is even legit. But how do you really tell if it's not to spec? Maybe you order something that is to spec and because of inventory comingling you get something that isn't. What then?
You can say that it's not the phone manufacturer's fault and you may be right but it is likely going to result in a lawsuit the manufacturer would have to defend. Was the phone up to spec? Was there a compliance issue? The battery manufacturer and phone manufacturers will each point fingers at the other.
Replaceable parts are huge sources of potential problems so as much as it annoys me (and it does) that I can't upgrade the RAM in may Macbook I sort of get why and how we got here.
[1] https://www.ibtimes.com/man-electrocuted-death-while-using-c...
And how to ensure battery quality is a completely independent problem from whether you can buy a battery from someone other than who made your phone. There have been fires with original batteries and there have been third party batteries without any problems.
It's like 1975 AT&T arguing that you should have to buy an AT&T phone because it connects to the phone network. Of course they want you to buy theirs, the issue is that they shouldn't be able to require you to -- or there wouldn't be any iPhones to begin with.
This is one of those things which is technically true but practically a red herring. The amount of weight it adds is on the order of grams and the cost is pennies. They stopped making replaceable batteries because they want you to throw away the device and buy a new one.
> and who is going to keep making them?
This is the least of the trouble if the model is at all popular, because the margins on custom batteries can be quite large (e.g. charge $15 for a battery with a $3 manufacturing cost), so as long as the specifications are documented you can expect somebody to be willing to make a buck.
But what would really help is to standardize the batteries. The shape of phones hasn't really changed since the start of the iPhone era and there are only meaningfully a half dozen different sizes.