Would it not easier just to integrate paypal in the iMessage?
No. Paypal comes with higher fees, certain restrictions and occasionally causes you a ton of headaches when it locks you out of "your own money".
For instance: Sending from Germany to my friend in the US 30 EUR. PayPal would just to exchange the EUR into Dollars. Circlepay would first exchange EUR into Bitcoins, do the transaction and then exchange Bitcoins into Dollar. What do I see wrong?
I don't see why they would use Bitcoin in between if the users are sending EUR -> USD or USD -> EUR. Have they actually stated something like this somewhere?
dosn't apple store ban all crypto coin related apps?
No, they don't.