Because they are trying to compete with coinbase? Verification is a pain in the ass, so if you know you get a free $50, thats good for them. It means, when the bull run comes and all the idiots start to buy crypto, they will make a lot more on than $50 from you (in fees and shit) in the future/bull run period. Plus they probably have some sort of deal with the stellar owners. So the $50 was probably allocated from the marketing budget. Just like you spend x million dollars for google ads or pay for a newspaper ad. They instead decided to give that money to its customers