Because they can set their price for the game.

I agree they can set their price for the game... soo....

After someone buys a game, they shouldn't be limited with what they can do.

That's your opinion of how this game should be priced, but it's just an opinion and obviously Valve think differently. (See above re: pricing.)

Hearthstone makes you pay for draft too, but they just give you the option to pay with time instead. Despite claims in /r/hearthstone we know that the vast majority (talking less than 10%, maybe as low as 5%) can go infinite just based on arena and must at least luck out and get other quests they can complete to keep playing for nearly free. Vast majority fail out, in fact IIRC like 50% of people in arena only get 2 wins a run.

For a lot of people, the option to play with time is definitely valuable, but I'm not sure I see Valve changing their mind on this since they said it's not F2P or grind to play.