Ok, some explanations seem fair..
1) Which command takes priority depends on the order in which they arrived in
your inventory (incidental to #2 - I thought worn/wielded took even higher
priority, but oh well). Since most toolbelts are autoloading, they get pushed
to the bottom of the stack.
2) All of the toolbelt functions depend on you wearing it; so 'weigh' simply
fits the pattern in that regard.
3) When the weigh command was first added, you could weigh items on the
ground... then I discovered that some people were taking advantage of that
functionality we 'weigh' people and find out all the items they were carrying.
I prefer the defensive option then - rather than try to guess all of the
possible bugs people can come up with, restrict the functionality, and add new
bits as needed. Indeed, this is why "weigh bench" and "weigh disc" were
created.

On a side note, I'm pretty confident that the toolbelt was the first item in
the game to allow you to directly weigh items, and if that's correct, than you
should take up the "duplicate commands" bug with those items coders

Although, really, different items give different bonuses in different ways,
this is just how the toolbelt handles "weigh" - although, I will still listen
to reasonable requests for new features; but like the "weigh items on the
ground" option, not all will be granted

Melemkor