Is it more irresponsible for an 18 year-old to take on six figures of debt or for the lending institution to so easily give it out?

From the standpoint of the lender, it's a little complicated. Financially, I'm sure it works out for them under the current rules or they wouldn't do it.

Socially, you could make an argument that they should be more careful. But to the borrower, any denial could be interpreted as taking away an opportunity, and could really backfire if not done very transparently and rigorously following an acceptable process. What if the process unintentionally has a disproportionate impact on one race, for instance?

For something seen as a ticket to the middle class, it's politically better to leave the decision up to the individual.