why spam emails are poor at spelling

It has puzzled me for quite some time why there are so many misspelled words in a spam. In fact, solely by counting the number of misspelled words one can reasonably dictate whether a message is spam or not. Later I learned somewhere that the very reason is that most spam filter programs counting on the appearance of specific keywords like coupon, free, etc. Now to evade the detection, they will intentionally misspell those words. But then a spam filter can use that information. So the game goes.