If my memory does not betray me arena always did cost gold, ever since it was introduced.
You contradict yourself.
Whether you spend your ore on kegs or Arena tickets is up to you; resource management and prioritizing is all it is. Besides, Arena gives rewards for the currency spent -- rewards that always include a keg.
How about if I say: Why is it an option to pay real money to play Arena mode? If paying real money to play Arena mode is an option, why not for Season mode?
That's my point. It just seems like the thin end of the wedge, which demonstrates the business model of the game. Play 700 hours and grind some small rewards or just pay real money for the chance of a good game.
Because Arena is an entirely separate mode; it has its own ruleset and special decks. Seasonal is just Classic with a twist.
And, again, Arena offers rewards that get better and better as you win more matches. Arena not having an entry fee would be ridiculously broken; free resources even without winning a single match.
MTG also had separate game modes when I played it years ago. The mode was free but I think that you had to buy cards to be able to play it. What percentage of players win enough games to make the "rewards" worthwhile? What is "worthwhile" anyway? I would guess that it's a tiny amount of people who get anything worthwhile in rewards for playing this mode. If that's right, then that's the business model showing itself again: get a lot of people to spend money and have very few of them get anything worthwhile for doing so.
I don't see why the devs can't scrap the rewards in Arena mode anyway. I'd play it. For small rewards, like you get in the main game.