It's a well-known industrial fact that people are much less risk-averse with arbitrary tokens than they are with money. Even changing the representation of the exact some money from physical cash to a number in a bank account has this effect.
There are likely other reasons or useful side effects to this practise, as alluded in another answer to your question and in the answers to this Economics SE question, but this is the most consistently stated reason for the use of value tokens rather than showing a currency value.
There are certainly no general laws forbidding a display of currency value in an account, as this is done regularly for all matter of services, from public transport to gambling websites to financial trading to education institutions.