Yes you are quite right, even if the student is above the minimum pilot weight, weight can still be placed in the aircraft to aid with first solo. This cause's less tail down attitude and an different take off and landing visual cue's that can be off putting. I do it where I work with 206's.
Unfortunately even after a thorough pre solo briefing, the poor little student may still do something quite silly at the critical moment. Standard judgement from an experienced instructor can get caught once in his career, as a student may display perfect ability and attitude, and then suddenly stack it.....