Roblox is a sandbox game sort of like Minecraft, but with way more power given to third-party developers and users to develop and distribute content in it. What you'd consider mods for other games are the whole point of Roblox, and MTX can be implemented by those third-party developers using "Robux" as a currency. The MTX can be your usual MTX fare of skins and power-ups, but also often abused for less ethical purposes, like gambling mechanics. The fact that Robux can be cashed out for real money via official and unofficial methods mean that's it's ripe for abuse.

Update: Also, I wasn't aware of this before the article, but apparently gambling sites have figured out a method to link a Roblox account to their external sites, and then use the user's Robux wallet as currency in their illegal online casinos.