Most of the bonuses are a trap either to attract new clients or to make the trader "think" its real money as bonus in his account. You only trade with your own money in case of any "deposit bonus" offer and no deposit bonuses are deducted when you withdraw any profit from such accounts.
I would rather say those deposit bonus like double your account with margin is definitely a trap. They actually didn't give you the credit, they just wanted you to open more position.
I think bonuses are trap for those witty traders expecting to get free money from broker. I understand its purpose as increasing your margin to reduce the pressure from leverage risks. Only uneducated trader will consider bonus as a solicitation to open bigger position as leverage possess essentially the same feature and none of us says that brokers offering high leverage solicit us to use its full power.
Now a lot of fascinating offers are released not only bonus funds but also for example 500 USD as a gift for new clients from freshforex broker. Thus, you may trade a significant amount without your own funds involved.
A modest bonus with clear and reasonable terms from a trusted broker may be ok. The problem is that even good brokers are pressured to offer large bonuses to compete for clients, and brokers don't make money by giving money away. Then there are the groups that hedge between accounts to try to effectively transfer the bonus from some accounts into withdrawable profits in other accounts. This leads even "good" brokers into creating bonus offers which are stacked heavily against clients.
Don't take a bonus unless you have a good history with the broker and trust the broker.
Don't take a bonus until you've carefully read and fully understand the terms.
If this is too difficult, then just don't take a bonus.