It made me look at my email with my account there and thought that I've received some bonus as well since there's this email that has "bonus" on the title but it's not the same email of what OP and bitbollo received. I guess it's for specific users that have received that bonus and email and lucky you are guys that stake gave you a bonus and free $40.
It is a bonus for what, they should state what the bonus is for, so we will not be guessing what is intended for and those who received will not wonder, there is always a qualification why they receive this bonus and I wonder why on Russian language, maybe because the majority of those who received are Russians so they send through the Russian language to save time and effort on classifying who to send, this buzz alone is a good promotion for Stake.
On the 2nd post of OP, it's not really an intended bonus as said but since they've already sent it to selected users, that means that they're free to keep it and they're generous on that part for those users that have sent the $40. We don't need to guess, those that are eligible for that might just be lucky to be selected by their system.