me every time

Inaccurate. The whole point of a free market is that you can buy whatever you like without those kinds of mandatory purchases

Is the joke implying that the purchase was mandatory or an act of free will?

It implies mandatory when it says "required" book

But in an unfettered market, each company does its best to compel you to buy only its merchandise (looking at you, proprietary Mac accessories).

Yes, they compel advertise and persuade but they never hold you at gunpoint to buy their Mac