What purpose would it serve to just give away 20 free orbs to everyone?

Create a happy userbase willing to spend more money on the game and recommend it to their friends.

People getting more cards means they probably want to play those cards, maybe they are more willing to buy some orbs to create decks around them. Maybe they want to play multiple factions and the 20 free orbs would allow them to do that.

The point of the Humble Bundle was to bring in new people.

That's what I thought as well, but I'm not so sure about this anymore. 20 orbs wasn't only given to new players, it was given to new accounts. If you had played for less than a month you are better of creating a new account. If you played for a week and spend $10 on start pack you are better off starting a new account. I had played for 2 weeks and spend $30 and was able to craft a couple good songhai decks that easily got me to diamond, but with the humble bundle I started 5 new accounts and now I can play all factions, seperately ranked with midrange decks or better. It seems like this is what CPG wanted, they wanted players to make multiple accounts, because that would look good on paper having their "playerbase" expand several times when every player has multiple accounts.