I. Freeware
First of all, the reasons why Spybot-S&D is free:

I.a. Dedication
Spybot-S&D is dedicated to the most wonderful girl on earth

I.b. Binary
What do you get if you buy software? Lots of ones and zeros, nothing more. If they were distributed as art, I could understand paying it. But if the main goal of their order is to earn money - by fees or ads - I don't like it!

I.c. Conclusion
This means that I grant you the license to use Spybot-S&D as much as you like. But if you like it, I ask two things of you: say a prayer for me (and the most wonderful girl while you're at it ) to your god - or whatever you believe - and wish us some luck.