This sentence in english seems odd,
Am I going to pay?
No, i´ll pay.
( the use of the contracted WILL future is most commonly used in this context)
Nein, Ich bezahle appears to refer to a doubt in an action in process.
are you going to the concert for free? nein, Ich bezahle.
I don't know about the usage pattern of English speaking people but in German the usage of present for things that are in the very near future is very common (and sometimes for things that are in the distant future or things that will never happen in the future again or…). Maybe this has to do with the extra effort of using an additional word that has more than one syllable („Ich werde bezahlen.“ – „I'll pay.“).
But your interpretation works, too.
"Ich bezahle," like any other present tense German verb I believe, can be interpreted into English in a few different ways. Like your example, it could be translated into "I pay," which does make the English sentence odd.
However it can also be translated as "I am paying," which makes your situation in English sound much better. "Am I going to pay?" "No, I'm paying."
There is a way to say "I will pay" in German but as mentioned by Binweg usually Germans will just use the present tense to mean something very near in the future (if you were arguing with a date over who was paying for dinner, you would say "Nein, ich bezahle" to mean "No, I'll pay.").
However, if you were saying you would pay for a trip you are planning a year from now, then you would use Simple Future verb tense... that just isn't taught until much later on Duo. XD
According to the standard German dictionary Duden, they are interchangeable in everyday language.
Technically, however, zahlen refers to the amount that you pay, while bezahlen refers to what you are paying for. E.g.:
Ich bezahle das Abendessen = "I am paying for dinner"
Ich zahle 50 Euros = "I am paying 50 Euros"
Here you can see why it's "colloquial" to interchange them. Technically, if you swapped them in the last examples, you would get:
Ich zahle das Abendessen = "I am giving you dinner as a reward"
Ich bezahle 50 Euros = "I am buying 50 Euros from you [i.e. with US$]"
Literally, "I will pay" is future tense and Ich bezahle is present tense. Duolingo likes you to match the tense (because that's what it's often teaching).
That said, German and English do both use the present tense to talk about things that will happen in the very near future (so you are correct in a real-world sense). Just keep this for your real-world speaking though, and don't stress Duolingo out with it too much!
Well, the specific historical reason that came about I have no idea, but grammatically it's because German does not capitalize any pronouns (and actually sticks to that rule). In English we have many, many exceptions to rules. We don't capitalize he, she, it, they, you, or we.... just I....
.....which could lead into a discussion on collectivisim vs individualism in European cultures vs Western cultures.... but I have a feeling that's perhaps a discussion for another time. ^_^