Why is "Force GPU rendering" disabled by default Developer options? Why is this feature disabled? When I turn it on, many apps and become snappy and awesome with smooth graphics. Seems like it'd be nice if it was enabled by default.

The linked article in this question says that hardware acceleration wasn't default up until 4.0.

Is the "Force GPU rendering" option then targeting apps that use API for 3.0 and lower?
When GPU rendering was first added, it was very unreliable. Sometimes it would be slower than software rendering, and there were some kinds of GUIs that it just couldn't work with. For this reason, it was up to the app developer to test their app with GPU rendering, and set an option in the app's manifest (the same place that it declares its permissions) to enable GPU rendering for that app. The option was mainly to make it easier for developers to test the effect of GPU rendering on their app. This makes sense because:


most users don't know or care about what GPU rendering is, and don't know whether it should be on or off for any given app; and 
the setting really needs to be per-app, not device-wide, because some apps just wouldn't work with GPU rendering turned on.


After further development, when 4.0 came along, GPU rendering got more reliable, so it became the default for all apps: it's now up to the developer to explicitly disable GPU rendering if it causes a problem in their app. (That's very rare now.) The "Force GPU rendering" option is vestigial and rarely used even by developers now, because it's already the default.