This is explained by the fact that at the time of the execution of the smart contract, he does not have access to the details of the addresses that have been transferred, where and where, and therefore he cannot claim that the user who calls the contract has paid the amount necessary for the operation of the contract. When tokens are transferred from one account to another, the token contract updates the balance of both accounts. For example, a transmission of 10 tokens of 0x2299. 3ab7 to 0x1f59. .