Because unicorns are dangerous.

Just something quick and silly that's loosely playing off one of the latest clips from the movie, in which they're demonstrating fancy gadgets to Earth ponies that will help protect them from those dangerous unicorns and pegasi. Comments made by Brandon Ott and Bretaña Burgos in an EQD Facebook post about the clip implied that Flurry Heart may be the reason for the distrust between the pony races, which gave me this idea.

Flurryheart by Racoonkun https://www.deviantart.com/racoonkun/art/Flurry-Heart-599224021

The Facebook post: https://www.facebook.com/EquestriaDailyMLP/posts/4859218154091455