Not everyone does it, though. I mean technically yes, it's required grammatically, but it's one of those rules that isn't always obeyed. It's a mistake but common enough to be aware of it.
In my opinion there is a cultural difference here: While in English, as I see it as a foreigner, if many people spell something wrong, it becomes an accepted spelling, in German if lots of people spell something wrong, it is still considered as wrong. There are even websites for common spelling errors and wrongly used words which you can link to in internet discussions to convince someone that/how their spelling was wrong, such as
> http://www.das-dass.de/
> http://www.dassdas.com/
> http://seit-seid.dassdas.com/
> http://wie-als.dassdas.com/
> http://einzigste.dassdas.com/
> http://standart-standard.dassdas.com/