This is an example of the difference in attitudes between hobby dancers who may compete recreationally, and the traditions and decorum of more dedicated competitive efforts.

To some extent, you could call any competing professional who has pro/am students out on the inconsistency of this (and there are very real costs to their partnership that arrise from it) but for economic reasons that is seen as normal, while multiple partners is seen as a hobbyist thing to be outgrown if one wishes to really focus on achieving something in a partnership.