It's for the safety of the youngsters, they said.
In the Harry Potter world, the conflict between those who believe that magic is the privilege of Purebloods and those who vehemently oppose the former is one of the centrepieces of the universe. Throughout the story, Harry and his friends fend off Voldemort and others believing that Mudbloods have to be eliminated.
However, a mind-blowing fan theory on Reddit suggests that tools that would indirectly squeeze Mudbloods out of the magic society existed within the system all along. And one of such tools was the punishment system for underage wizards using magic outside of Hogwarts.
The two-strike system – when the first strike envisages a notice from the Ministry of Magic and the second one takes your wand away – does not appear to be a fair practice when it comes to wizards who are born in Muggle families. First of all, it would seem that the punishment for using magic outside school as a kid would be a permanent ban on the use of magic in the future – something that would effectively strip someone of a chance to become a wizard ever again.
Secondly, it seems to target Muggle-born wizards precisely because of how the violations are policed. After someone receives a first strike, the Ministry of Magic starts to track them, but it's not the child who's being tracked, but the entire radius around them. If a spell is cast by an underage around other wizards, the Ministry would not know who exactly cast the spell.
This means that a Pureblood could possibly escape punishment for using magic outside school if they are around other adult wizards – for instance, their family members. But when it comes to Muggle-born children, they would be the only magic users to track down in the given radius. Moreover, they might even be framed – something that happened to Harry when Dobby cast a spell, because he was the only known potential magic user in the household.
Two factors combined, the punishment system basically made Muggle-born wizards more vulnerable to the strikes from the Ministry, making the chances of them getting stripped of their wands more likely than those of Purebloods, the fan theory concludes.
Fans were mind-blown when they looked at the punishment system from that point of view – even though some were ready to suggest that the Ministry might not have meant to create a system that would deliberately target the Muggle-born.
"I'm 100% on board with this! I think there's a possibility that this isn't a purposefully act by the Ministry though, and that instead the Ministry thinks these rules are fair, just because it is too pure-blood centric to realize that these rules disproportionately affect Muggle-born wizards." – /ApocryphalShadow
Whether planned by J.K. Rowling or not, the fan theory might be an interesting yet chilling example of how the laws in the magic world may coincide with those in reality – unfortunately, in a pretty depressing way.