Presumably BAA doesn't really want to get into the business of determining which courses are "too downhill".
The easiest cutoff would be to use the 3.27m/km for Boston itself, which is what USATF uses for determining OTQ eligibility (technically they say 3.30m/km). But net start to finish downhill isn't a perfect measure of overall difficulty. Boston itself is known for being somewhat tricky tricky despite being ~460 ft net downhill - I think for for folks not used to hills, you could argue its harder than a flatter course. Its certainly a harder course than CIM, despite CIM being only ~340 ft net downhill.
And then there's the whole PR angle. As much as LRC wouldn't care, they would by defacto be crippling if not killing all of those races, which isn't a great look. Plus its a bit of a wierd look for a downhill marathon (which I'm sure that wants to keep their image of being a tricky race) to claim other races are "too downhill".