Most casinos restrict the maximum bet size for players who play with bonus money. Without the max bet rule, bonus abusers could get a bonus from a casino and try to quickly multiply it by placing big bets. If they succeed, the multiplied balance would allow them to easily meet the original wagering requirements and cash out their earnings. The max bet rule is therefore a valid rule used to prevent most extreme cases of bonus hunting.
So to speak, a player who is playing with a bonus with a maximum bet of €5 is not allowed to place a bet higher than that. If the player breaks this rule, their bonus and/or winnings may get taken away by the casino.
The maximum bet rule is an industry standard in online gambling used by casinos to protect themselves against bonus abusers. Most casinos don't technologically enforce their max bet rules. This means that the players are able to place bets higher than they are allowed to according to the Bonus Terms and Conditions (T&Cs), so they have to carefully watch their bet sizes themselves. Also, be aware of no verification casinos, as they have uncommon bonus rules (if they offer bonuses at all).
Many online casino players know about this rule and are careful not to break it when playing with bonus money. However, there are still a lot of players who are not aware of the maximum bet rule. These players might unintentionally break it. This often happens after a big win, when a player might naturally increase the bet sizes because of the increased bankroll. In these situations, players feel cheated if their wins are taken from them later because they weren't aware they'd done anything wrong.
It is a well-known fact that most players do not read the T&Cs, however, most will follow the rule if it is visibly stated to them in the casino lobby, in the game itself, on a prominent part of the page or in an advertisement informing about the bonus.
In an ideal case scenario, casinos should enforce their maximum bet rules at software level and therefore not allow players to place higher than allowed bets. If enforcing max bets is not possible because the casino runs on third-party software that doesn't support them (or for any other reason), the casino should carefully judge each case separately. Some players may break the rule on accident, e.g., by clicking the 'Max Bet' button on a slot machine by mistake, so it is unfair to punish them for this.