Economics question?

this is the part from my text book.
1984; The supreme Court ruled that as long as a bank does not offer investment advice, it can provide discount brokerage services. Subsequently, appeals courts have ruled that a bank subsidiary can offer advice.

why couldn't the banks offer investment advice? could you please tell me why? what problems would be caused if the banks can offer advice?

Simple answer: conflict of interest. Banks do not make as much money by loaning out money as you would think. Most of their income is derived from investing. As a fellow, and possibly, competitive investor, they could help themselves by encouraging or discouraging certain funds or stocks that they themselves are interested in.
One hypothetical for example: They want to invest in Do-dad stock. It currently sells for $2 a share. The president announces to all the "advisers" to push that stock on March 15. At 5:45 p.m. EST (15 minutes before closing), the bank buys 1,000,000 shares for $2,000,000. The next day, because of the advice of all the "advisers," another 1,000,000 shares are bought up throughout the day and the price rises to $2.10/share. That doesn't seem like much to the guy that bought 10 shares and only sees a profit of $1 (10 x $.10) but the bank is (literally) making bank. In one day, they made $100,000 (1,000,000 x $.10).
The idea behind the rule was to remove the temptation that banks could have to benefit themselves. If you look at the study of economics as not being the study of money but as the study of what makes people make the choices they make, then this seems like a wise rule.
I wonder why they took it away and how many banks decided to use this to their own advantage.

banks and brokerages used to be the same business. they were divided after the great Depression started to prevent abuses that had occurred. [depositors' money had been used to fund bonds and stocks held for sale to investors, even after it became clear that investors weren't going to buy the securities.]

i think this was part of the Securities and Exchange Act of 1933 and maybe I'm in error about that.