[imauld]

> is it morally defensible to quit a (perfectly suitable) internship, due to the grass being greener somewhere else?

Yes, it is. If that company decided tomorrow they needed one less intern they wouldn't hesitate to cut OP or another intern loose.

This is a business transaction. OP is exchanging their time for money and in return the employer is hoping to extract more value from the employee than they put in. If either side of that transaction feels they aren't getting the value they want from the arrangement it's in that parties best interest to move on. The OP hasn't started working there yet and isn't scheduled to start until June, that is plenty of time to find a replacement. There is no shortage of capable CS students in the world.

Additionally if the OP just isn't interested in the work it's not really in anyone's interest to have them there. OP shouldn't be stuck working at a place they aren't really motivated to just because of the timing of the offer. I'm sure any manager would prefer to have an interested and engaged intern rather than just going through the motions.