Did you hear the part "could" which can be next week, next month, next year or never. If you know the way bank work, you would realize that they cannot just wake up then decide what they want to do with customers deposits. Except they finance another company to carry such activities with no relationship with the bank and enough provision to cover for the possibility of it failing, they cannot pull it off. Every cents in the bank is accounted for and monitored from the Central Banks, deposits commissions, fraud units etc.
The point is, its not that its impossible but the allowance to use deposits of customer to engage in crypto currency and be open about it, has to come from the way top which we all know its not something that is easy to come by. Its a good proposition and I hope to see it happen anytime soon.
Its true that banks have to report everything to central banks but that does not mean that banking sector cannot enter crypto market, if you are aware the biggest bank in Saudi has adopted ripple. Banks basically invest in different sectors like real estate, stock market etc other than providing loans to consumers which generates income for them. Investing in crypto may be the other choice for banks, its possible that banking institutions may start investing in crypto sooner or later.