Voting on changes to the Rules.

I can't see any logic in it, no matter how hard I try.

I killed it 5 times ;)
If you really think about it and look for logic, instead of just dismissing it based on your personal opinion, you can find some, but in this case, you'll have to add a couple of clarifying points:
1) Why restrict something that the developers came up with? Where is the element of randomness?
2) However, the number of monsters is something we can discuss. I personally think that +5 monsters of level 7 is a bug, and it's been present since the first Heroes game (those familiar with the first game will understand that this bug is more pronounced there).
3) I think it's perfectly fair to allow buying only the extra growth that corresponds to the base growth of that unit (2 for all level 6 monsters, and for Phoenixes, and 1 for the rest of the level 7 monsters).
4) The probability of a specific monster's weekly bonus appearing is approximately 1 in 140 (9 castles x 14 types of creatures).
5) I believe that the weekly bonus affecting monster growth appears more often on week 211 than on the 3 preceding weeks (121, 131, 141).
From points 4 and 5, as well as the fact that there are a large number of weeks that don't offer any bonuses, we can calculate the true probability of a specific monster's weekly bonus appearing, which is many times lower than the chance of getting a bonus of 1 next to the castle.
Conclusion: Of two evils, I choose the lesser.