Why does the mazoku barrier let people not use magic outside of it?

Question

It is stated that people outside of the mazoku barrier do not know any magic. Why is that? Shamanistic Magic works by using the forces of nature, which should work outside of it.

Answer 1

I watched all of Slayers sometime in the last four years, but unfortunately, I do not currently have access to the episodes. Thus, I have been making do by reading the Fandom Wiki pages for synopsis of episodes and other concepts in order to come up with an answer.

Answer to the Question

The answer to why people outside of the barrier are able to not use magic is actually the same reason people inside the barrier are able to not use magic. There is no compulsion to use magic. In order to use magic, one has to cast a spell. So it's obviously a choice whether one uses it or not.

Analysis

I will now address other points brought up in the body of the question.

According to this, the Mazoku Barrier was a barrier that was created by the mazoku (beings who are invoked when casting some types of magic), and it completely sealed off the world we saw in Slayers from the outside world, up until Lina killed Hellmaster Fibrizo (this is explained in episode 1 of Try).

It is further stated in these articles that Holy Magic could not be cast while inside of the barrier, but could be outside. The article on Holy Magic states:

However, it is assumed that it will take some time for humans to rediscover the traditions and spells that were lost 1,000 years ago.

[...]

In order for humans to use holy magic again, they would need to translate the spells, now in dragon language, into literal human terms.

We will see why this is relevant soon.

The synopsis for episode 3 states the following:

When Gourry asks how they got into such a ruckus by simply walking around town, Zelgadiss and Amelia mused that the townspeople seem to be surprised by magic. Filia replies that of course they are, since the high concentration of magic within the Mazoku barrier doesn't happen without; the sorcerers in the outer world can only cast Lighting and Sleeping spells at most.

First of all, the world inside the Mazoku Barrier was completely sealed off from the outside world. There is no reason to believe the outside world would have the same knowledge of magic, including of Shamanistic Magic. Secondly, it is stated there is a higher concentration of magic inside the barrier, and this may have made people inside more attuned to it, and thus magic may have developed more swiftly inside the barrier while it stagnated outside. And third, the statements about Holy Magic gives us some insight into how spells can be lost and how long it could take to recover them. In spite of Holy Magic now working again, it doesn't mean everyone is going to be instantly using it. The same is true for all the other types of magic.

In short, just because Shamanistic Magic would work, doesn't mean the people outside have a good understanding of it. They seem to only use a little White Magic at most (which is the type of magic Amelia mainly uses).

Further Reading/Sources

https://kanzaka.fandom.com/wiki/EP01_(Slayers_TRY)

https://kanzaka.fandom.com/wiki/EP03_(Slayers_TRY)

https://kanzaka.fandom.com/wiki/Mazoku_barrier

https://kanzaka.fandom.com/wiki/Holy_magic

https://kanzaka.fandom.com/wiki/White_magic

https://kanzaka.fandom.com/wiki/Black_magic

https://kanzaka.fandom.com/wiki/Shamanistic_magic

https://kanzaka.fandom.com/wiki/Amelia_Wil_Tesla_Saillune