From personal perspective:
is it normal? yes. it happens fairly frequently.....
is it technically fair? Yeah, I would say so... some people take diplomacy in this game very seriously, and technically this is just a form of diplomacy.
is it acceptable? For some yes, for many, no.
The developers are currently working on an end game which might partially fix this... Since there is no crown sharing, only one alliance will be able to win. A merger would still be possible - however, it would have to be on a smaller scale. Merging two full alliances won't work - it'd have to be two half-full alliances, or otherwise some heavy cuts. It would still be possible for the best of two alliances to merge together and leave everyone else behind, though. Which, again, would be fair - but probably frowned upon. The developers just wanting to get rid of crown sharing, in itself, might tell you a bit about what people think about end game merges and pacts. It's a big enough issue for people that they want to start to change it.
The question of is it acceptable - most people who don't find it acceptable find each other..... and create alliances that live by the no-pacting philosophy, or something similar to it. The easiest example of this is that I can direct you to is in the acropolis: see Curtain Calls in the pre-mades thread, but there are other very well known alliances that have vaguely the same philosophies. I wouldn't be surprised if you found a few players in your own alliance who feel the same way as you and aren't happy about the merge.
I think one small issue sometimes is that things come to a bit of a stalemate at the end game... and for the purposes of closing out a world sometimes certain alliances will..... stray from their beliefs? just so they can close out a world. Some people will do anything for the win - or, otherwise, just to make sure the other alliance loses.
I honestly think there is a big theme in grepolis now where a lot of players recognize each other, and there is a lot of bad blood between certain players or groups of players. I've spoken to certain players who have directly told me that they joined a world just to rim a certain player, and was going to leave immediately afterwards. People are more than willing to throw away their morals just to make sure someone else doesn't win.
I think it has been normalized and people are used to it, but i'm not sure it's been fully 'accepted'.