Why does Philadelphia today plan to decriminalize the use of small amounts of pot and isn't this backwards?

The "war against drugs" isn't working - just as prohibition didn't work in the 1920s. Prohibition was a failed experiment, and in the end alcohol had to be legalised; prohibiting drugs is also doomed to failure.

If drugs were made legal then they could be sold at a specified purity so no one could accidentally overdose, taxed at a very high level, and the money gained from taxation could then be used for rehabilitation and education. At the same time, all the gangs that raise billions each year from drugs would be put out of business.

BTW, I'm not a drug user myself and I think using drugs is just stupid, but I do know that when something isn't working then it's time to stop.

Bro, let me give it to you straight. All of your assertions are based on the illogical fallacy of legalization. Even if it were legalized entirely, your sweeping statements would not be true. Decriminalization simply means the fine will be less severe for small amounts. There will be no changes for the large quantities that gangs usually deal with. The provision will make the sentencing lighter on a college student for example, who just smokes every now and then.

"If more people start using pot because they do not fear arrest then demand is going to go much"

^^ The above logic has been disproved time and time again. Look at countries with no drinking age, or amsterdamn. The proof is in the pudding.