Some of the online casinos will change the betting limits within the casino software when you receive a free bonus. This is a part of the terms & conditions of the bonus that you redeemed within the casino. The betting limits were likely changed to make sure that you playthough the free bonus to meet the wagering requirements over a normal period of time. Some players like to take a free bonus and use the maximum bet in order to quickly fulfill the wagering requirements and cash-out the bonus. This helps to protect the casino from bonus abuse and fraud.
Also, the online casinos typically change the betting limits for the table games if you are permitted to play them with a free bonus. Some of the casinos permit play on the table games but with betting limits and without the wagers being added as a part of the wagering requirements. Make sure that you always read the terms and conditions for a free bonus, because the online casinos will often restrict certain games, limit the betting options, and set specific wagering requirements to put the odds in their favor and reduce the risk of bonus abuse by new players.