All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.
In reality, these bonuses that cannot be withdrawn but which you can benefit from by trading only constitute a money leverage system because what is taken into account are the facts and meanings, not the names. Therefore, you are not entitled to accept those donations (bonuses).
As regards investing through that system while avoiding the Islamic prohibitions that it involves, then, if this is really possible, there is nothing wrong with it.
Among the requirements that should be met while using that system is to avoid borrowing money from the broker even without usurious interest and to fulfill the conditions of lawful exchange including the terms of buying and selling on the spot (hand to hand), even virtually by something that serves as and takes the ruling of receiving cash and exchanging hand to hand (like a bank draft or check).
For more benefit, please refer to fatwas 262402 and 88475.
Allaah knows best.