Charlie did it to make him stronger, let give the list of possible heirs
Odysseus (or whatever): Averages Joe, indecisive, possible he grownup with everything being handed to him and is apparently useless.
1st princess nobody: same as Odysseus.
Schneizel: the perfect heir for the throne, intelligent, ambitious and smooth with the ladies. Having older siblings probably help him by havening to compete for attention and have someone to lookup.
Cornelia: almost the same as Schneizel but with more focus on military, the main and most important difference is his sister, his weak point that is several times exploited by Lelouch and Schneizel; this leaves her as a good heir for the throne but with a weakness that makes her easy to manipulate.
Lelouch: Same as Schneizel and even showing greater potential but with the same weakness of Cornelia.
Nunnally: blind and cripple after the incident, No fucking way she will have the throne
Now let see the situation: I have a good heir to the throne that’s being a little bitch lets use him and at the same time put him in a challenging and difficult situation where he may die or become stronger and more capable heir if he dies have Schneizel ready to take the throne when I kick the bucket.
There was some “for the lulz” in the decision but you can’t tell me it doesn’t makes sense.