I take it you are going on the fact it says you possess one? I took have you get one but you still need to buy the thing from your starter gold and it didn't just appear before you when you decided you became a wizard. It's a tool of your trade much like a sword is for a fighter and they didn't get that for free. The cost is comparable to a fighter getting his weapons and armor and if you don't need to purchase it, it does seem a bit unfair to the other characters that require higher priced armor/weapons.
Possess means you own one, not that you can buy it. Anyone can buy a spellbook, the wizard gets it for free. I'm not debating whether its fair or not,
the text is pretty clear that the Wizards owns his spellbook and that the Cleric owns a ritual book.
I make my players pay for the spellbook, and I always pay for mine. Otherwise, Wizards start play with 70g unspent while others who've had to pay for everything start play with 5-15g.
Do you also make them pay to buy the rituals themselves? All of the the level 1 rituals in the PHB cost 50-75gp, no way a level 1 character could afford 2-3 of them in addition to other gear.
As for that 70gp the wizard has? Thats what he uses to buy components to cast the rituals.