Okay the idea behind it is so that you dont get too harsh of a fine for just personal use. If you can sit down and kill 3 grams shy of an oz in one sitting, you are my hero.
Basically they draw an imaginary line that separates personal use and sale. Sure 25g can be looked at as pretty low, but for your average toker you generally dont carry more than a few grams at a time if you are just keeping it on you for personal use.
Have I had more than 25g on me before for personal use? Sure but its not too often. Also if you pick up more than 25g you just split it up and have a passenger hold onto enough to make it so you are both carrying a smaller amount.
Here is where NY gets fucked up though. If you have a pipe or something along those lines on you and an officer finds it you can get charged with a misdemeanor.