In Ontario, the legal basis is section 137 of the Securities Act:
Rescission of purchase of mutual fund security
137 (1) Every purchaser of a security of a mutual fund in Ontario may, where the amount of the purchase does not exceed the sum of $50,000, rescind the purchase by notice given to the registered dealer from whom the purchase was made within forty-eight hours after receipt of the confirmation for a lump sum purchase or within sixty days after receipt of the confirmation for the initial payment under a contractual plan but, subject to subsection (5), the amount the purchaser is entitled to recover on exercise of this right to rescind shall not exceed the net asset value of the securities purchased, at the time the right is exercised. R.S.O. 1990, c. S.5, s. 137 (1). [...]
(5) Every registered dealer from whom the purchase was made shall reimburse the purchaser who has exercised the right of rescission in accordance with this section for the amount of sales charges and fees relevant to the investment of the purchaser in the mutual fund in respect of the shares or units of which the notice of exercise of the right of rescission was given.
You are not allowed to recover more than what your purchased assets are worth at the moment of withdrawal, except for sales charges and fees charged by registered dealers which are recoverable separately.
It can be disadvantageous to the fund because there are other costs incurred when the rights are exercised (but that's part of their business risk), but it does not allow the purchaser to profit.