I wonder if some students feel (or have been told) that actually posting their code violates an academic standards agreement (sharing their work to date with everyone else in the class, after all), even though asking for help is explicitly permitted...
It might be that they are afraid that:
- other people see their coding quality
- other people (re)use their code
- teachers can see where their code is fixed (iso doing themselves)
- or they are just lazy
- ...
Don't teachers have access to software to check if things like English coursework has been lifted (plagiarized) from the net. Maybe this also extends to software where if you post it online for help then other will use it and when you come to submit it's flagged as copied code?