This is really difficult.
Both 「X とあって、Y。」「X とあっては、Y。」 can be a valid sentence depending on its context.
Firstly, 「X とあっては、Y。」expects that usually "it is not Y." But, because of X, it is Y.
It is expected that a meet of a minor sport is not highly guarded, but the speaker finds this is exceptional because this is world wide. So, it is highly guarded.
On the other hand, 「X とあって、Y。」 does not refer to usual cases.
It is highly guarded. It is natural because a lot of VIPs come from all over the world.
Let's get back to the original sentence.
「さすが」 and 「どの試合も目が離せない」give the context.
「さすが」 is an adjective which means "there is a fact which proves something is as good as expected or as a reputation, and the speaker is impressed."
In this case, the fact is that 「どの試合も目が離せない(not boring at all)」.
Let's put とあっては and check what it means.
- とあっては implies: It is expected that every game is boring. But, this is exceptional because this is an international cup. So, it is not boring at all.
- さすが implies: Also, the fact that every game is not boring proves the international cup is as great as expected. The speaker is impressed about it.
So, OK, did you expect the games are great or not? There is a contradiction.
On the other hand, 「とあって」 does not have this problem.
- とあって implies: No game is boring. It is natural because it is an international cup.
- さすが implies: The fact that every game is not boring proves the international cup is as great as expected. The speaker is impressed about it.
They are consistent.
「さすがに」 is similar but different. It means "there is a fact which makes something exceptionally as good as expected or as a reputation, and the speaker is impressed."
In this case, everything is consistent.
It is expected that every game is usually not that interesting.