Nowhere in canon does it say that karma is a requirement for learning a spell. However, the rules are left to the GM and players to interpret. Keep in mind it still takes time and money to learn new spells. It's not as if people who design spells do it for free. To hire an instructor it would take at least 1,000 nuyen per force not to mention a bonus for the teacher's Instruction active skill. Also, it takes one full day to learn a force 1 spell. A training day in Shadowrun is roughly 8 hours. So, you'd have to pore over the formula for 8 hours before you could learn it on top of spending 50+ nuyen per formula and then another 50-200 nuyen simply for the fetish. The average magician doesn't necessarily have these resources, let alone would the exact formula be available with the right kind of fetish for that formula.
Addtionally, if you follow standard character creation rules, you can still get 30+ different force 1 spells if you wanted to, so I don't really see much difference in paying 1 karma for an un-limited spell and spending no karma for a spell which requires you show off the fact that you're a magician by fiddling with some gewgaw before you throw the mojo, have to pay extra for, and simply have to find available.
Again, interpret how you will, but nowhere in any canon that I find does it elaborate on the cost of force 1 spells with limits.