Hi Rudolf,
I'll let the developer members of the forum answer the "why" questions. I can run a SDK debugger, but stake no claims about understanding everything that is going on within the Android OS and what the implications are for feature availability. &: )
I'm most familiar with Symantec products, but a bit of Internet searching shows that real-time / auto-protect is not standard among all the Android AV products on the market. Auto-Protect is included on the Symantec agent for Windows Mobile which comes with SMS 7.2, but the engineering behind that OS and Android is completely different.
Here's a couple screenshots, BTW, of SMS 7.2 on Android picking up (during a manual scan) the EICAR files downloaded from eicar.org: first discovery the confirmation for removal....
And then a "no problems found" screen after removal: