My school just started grading legal writing, and I agree, it is a pretty tier three move. 1Ls are under way too much pressure as it is, there is no reason to add to that pressure by putting a grade on something they have typically never done before. High pass/Pass/low pass creates enough of an incentive to do well.
And, if you did poorly on your first memo, read the professor's comments and figure out what resources are available to help you improve. My school has a legal writing office where you can take your papers for feedback before submitting. It is an invaluable resource and is the reason I ended up with a high pass despite my terrible grammar and fifth grade level writing. Also, dont stress the grade too much. It is your first law school grade, and it is in a skill set you have no experience in yet. Just take it as a wakeup call that law school is probably going to be harder than you anticipated. Bust your butt from here on out.