If we make a noun phrase in which scissors is embedded as a non-head position, we take out the s.

We want to hear "scissor sharpening service", rather than "scissors sharpening service".

The latter is not considered grammatically incorrect, but it lacks euphony.

In some cases plural-sounding words can't be avoided in the middle of noun phrases, like "denotational semantics lecture". Semantics is a singular that people treat as a plural sometimes ("these semantics are ..."). We can't delete the "s" to make semantic, because then we get an adjective. That can be in the middle of the noun phrase, but it changes the semantics. A semantic lecture isn't one about semantics but one that has semantics (about any topic whatsoever).