The term 'stamp' is a metaphor. What is affixed to Keychat messages is not a specialized stamp, but rather generic Bitcoin ecash. 

Why does the traditional postal system require stamps? Why not just affix cash or paper money directly onto the envelope? It's likely because the cash could be stolen, and the glue might damage the paper money. nostr:note1g7v8x5dzy45q9jkxhug5h3sykrldl40zvpuyzfgnmsajuntxxcksgdzekc 

 Most messaging apps send messages for free currently.  Would your relay list dictate whether or not ecash is required to send a message?

Is this stamp aspect more of a future proofing? 

 We are inclined to charge for our own relays, one satoshi per message. Users can add and use free relays on their own. We are curious to see what kind of situation this will create.