Logically, it would have been illegal to trade between Equestria and a country that had chosen to isolate itself and thus prohibit all contact with its neighboring country. However, as seen in reality, it is not possible to completely isolate oneself from the outside world, and pleasure goods such as cider (or hard cider) will have been sought after as smuggling goods.
 
It is also logical for the drivers to arrive the fastest so that one can gain the most luxurious customers who would buy the first and best without waiting, thus determining the market for the smuggled goods. So the ponies who competed with each other to be first out, thus had a lot to gain from such a business on a stretch of road going through demanding terrain.
 
From the comic # 88 it is possible to see that there are actually several roads instead of just one, most on bridge-like structures, meaning that Yakyakistan and Equestria had been closely linked to the latter chosen to isolate for a reason that remains unknown .

@FancyCat2008  
Does this man it used to be illegal to sell Equestrian hard cider to Yaks?
 
Wait, didn’t the ponies know next to nothing about the Yaks until very recently? I know, I know, it’s only a comic.

Daytona, NASCAR - very American. But the sport here described is based on the european rallying since specialized vehicles are used on ordinary roads although the name, “Twisty Mountain Roads” is interesting. There is not much that is mapped between Yakyakistan and Equestria on the map.
 
Since Yakyakistan had been isolated for a long time, it meant that “the daring drivers” were smugglers in the beginning. Cider smuggling was certainly lucrative, and the high speed may be to cut time to win as many customers as possible. And, of course, to avoid border guards looking for them.
 
In the later period when the smuggling became legal, the race should occur as the competitors would attract the public and thus advertise their companies. More recently, private drivers were allowed to participate as with Rainbow and Big Macintosh. But since an important amulet was stolen from the dragons, it is certain that they will track down Big through the doll Smarty Pants.
 
So the dragons will come and take up the hunt for Big as the race is still on the mountain roads. Raindow can get more action than she wants.

@Background Pony #B80F  
I knew someone was going to do that.

Dash ! Rude !

That’s a very… kid-friendly ponification of the origins of NASCAR, lol.

*throat clear*
 
Ahem
 
Dray To~na! Let’s go away!