As is usual with files which consider themselves important the default full permissions are for System and Admin only whilst Users only have Read and Read & execute permissions. I'm no way knowledgeable about such matters but it appears that if the System is set as the Owner even if your have admin rights the System settings prevent you from modifying or deleting it.
If you think about it the AV exclusions.ini is important - if it could be modified by an outside source easily then malware could exclude itself from AVAST scans.
You could change the permissions or more likely the Owner status using the Properties > Advanced if you really wanted to so your user account had full permissions for that file. But I
strongly recommend you do not.
The answer to your second question: it is because AVAST do not want you or anything else to fiddle with it accidentally or deliberately for the reason previously stated.
It might not be a Global Exclusions that added it to the the exclusions.ini, what about the various AVAST shield Settings > Exclusions? Have you checked them?
The AVAST exclusions list seems to be a bit flakey, I've had the whole thing reset to default (no exclusions) when, apparently, adding too many entries. I do not even know if this is what did it for sure but that was the only explanation offered.