Because libraries are absolutely brimming with computers. In Los Angeles, the libraries tend to have a clutch of general computers, a significant fraction occupied by the homeless (that's what I'd, can't fault them).
I don't see why you'd do this, though. Readers will continue to fall in price and the expense of providing them to the poor is partially offset by using them for textbooks (both that we no longer need to supply them out of "library" budgets and that we reduce textbook costs). It might be possible in major cities to fund readers for the poor out of the liquidation of branch library real estate in favor of wireless access points or whatever.