Please stop making this negative, this is really positive. This means that developers have the option to continue to support users that have older version of windows phone. Although everyone is getting mango for free, many users may and will continue to use NoDo or even earlier versions of windows phone. Why well a number of reasons
1. no computer
2. carrier block or slowness
3. user buys new phone that was sitting in a shelf for a while
Right now if all developers update their apps to mango everyone who currently have NoDO or below will have an empty store, with no apps to download. So many developers have Mango app updates ready to roll but they are not because their apps will no longer be available to current user base.
By MSFT allowing developers to have two versions of their apps one NoDo and one Mango all end-users benefit as well as developers. Yes fragmentation may occur over time as developers don't update NoDo versions but again mango is free and to ALL phones.
Android in the other hand has way to many versions out there and in some cases phone like the moto i1 have v1.5 and will never get an update.