At that point why don't they just send Tether or USDT (or another stable coin), why even risk playing with such volatility, can't these stable coins be transferred with few fees as well?
It definitely makes sense for them to use Tether/USDT for sure, I agree there. But making Bitcoin a legal currency is also a political and economic statement that goes far beyond simply facilitating digital payments.
There's hardly anything truly "bitcoin" about El Salvador's system.
https://www.coindesk.com/business/2021/06/21/strike-is-phasi...
This has been suggested as one of the many reasons Bukele is so keen on Bitcoin/Chivo: he can't print USD, but having control of the national wallet system gives him countless avenues to manipulate the system.
Of course it could also allow the government to do worse things like backdoor wallets and confiscate crypto.
There are lots if wallets not registered with the El Salvador government, and it will be impossible for the government to distinguish those that are genuinely foreign from unregistered wallets used by people ot for purposes nominally subject to El Salvador’s laws.
So, no, it won't make tracking any easier, nor wil it make taxes and tariffs easier to enforce generally.