[MarioMan]

I'm hoping this isn't a dumb question, but why does it matter that a public key is public-facing and unencrypted?

[jurgemaister]

If someone intercepted the communication, they could swap the Avast key for their own, allowing them to decrypt your message.

[Leace]

What jurgemaister said. If you don't have another trust mechanism (like Web of Trust) to validate if this is a correct key then HTTPS gives at least some assurance that no intermediaries between you and avast changed the key material.