aren’t demo accounts created for this purpose?
Bonus usually works as additional margin only, so it will be better to read about the conditions of the bonus first before accepting it. For example some brokers offer deposit bonuses and if this bonus is received, there will be no margin call
There are many regulated brokers that offer bonus options. However, just so you know, you can’t withdraw the bonus amount. It’s used just to give you leverage in case you’re falling short of trading capital.
Yes most of them are intended to either stimulate trading activity (boost volume and hence commission) or attract new clients, i.e. give them ability to test real trading conditions risk-free. However I don’t see anything wrong with that.