Without odds, players can't make informed decisions about their spending. They have no idea whether a rare drop is 10%, 5% or 0.002% of the time. So this regulation will help them calculate the probable cost of acquiring the item.

Example: If it costs $0.99 to open a box, and the desired item drops from 4% of boxes, then it will cost you $9.90 for a 33% chance of getting it. But if the drop rate is 0.5%, $9.90 only gives you a 5% chance of getting the item. At some point you might just decide to buy the item on the secondary market (and indeed, these odds will influence secondary market prices).

(I personally think that these loot crates are effectively gambling. Even if the developers claim the items have no monetary value - because they have to, otherwise they're clearly offering you odds of a return on investment - the same developers often operate secondary markets which assume the items have a value, e.g. Valve and its Steam trading service.)