Hi there,
My first post on this forum which I stumbled across.
As confirmed already there is no law preventing someone applying for a WV onshore having arrived on a VV.
In my experience this question seems to arise from individual cases where a person came on a VV saying one thing (e.g. holidaying) but then applied for a subsequent visa and gave information that seemed to contradict what they said before (e.g. provided a job offer that predated their visit). In such cases their intentions may be seen as not genuine without a good explanation.