bi- means two, as in bicycle: two wheels (circles)
semi- means half, as in semicircle: half of a circle
The problem is that the prefixes can be parsed as affecting either duration/interval as in (bi-week)ly, every two weeks, or frequency as in bi-(weekly), two times weekly. The same applies to semi-.
Personally I find the frequency interpretation a bit of a stretch—"two" is not the same as "two times" or "twice"—so I would tend to read e.g. bimonthly as every two months rather than twice each month.
The EULA also prohibits using Nightshade "for any commercial purpose", so arguably if you make money from your art—in any way—you're not allowed to use Nightshade to "poison" it.