What "bonus max bet" comes down to is simple: spins are worth less than they look. Free spins are the most common casino offer and the hardest to value at a glance. Each spin has a fixed stake set by the casino, often the smallest the game allows, and the winnings may land as cash or as a bonus with wagering. The number of spins in the headline says little until the spin value, the qualifying games and the terms on winnings are clear.
Spins are tied to one slot or a short list of them, chosen by the casino. The qualifying game decides both the return and the volatility of the offer: spins on a volatile slot mostly pay nothing and occasionally pay well, while spins on a steady one return small amounts more reliably. The game is named in the offer, and its return can be checked in its help screen before the spins are taken.
Winnings from free spins usually arrive as bonus funds with their own wagering requirement, often counted on the winnings alone. Some offers pay spin winnings as cash with no wagering at all, and those offers tend to give fewer spins. A requirement on small winnings can still take a long session to clear, and the balance may run out first. The terms state which of the two applies, usually in the line that describes the spins themselves.
A cap on free spin winnings limits how much the spins can turn into, however well they land. Anything above the cap is removed when the bonus converts to cash. Caps are common on spins given without a deposit and can be low enough to make a large hit on the qualifying slot pointless. The figure is part of the offer terms and matters most on volatile games, where the rare big result is the main reason to play.
Paying spin winnings as cash with no wagering costs the casino more per spin, so those offers tend to include a smaller number of spins.
Often a day or a few days after being credited, and any spins left unused simply disappear from the account.
The casino ties spins to one slot or a short list, named in the offer, and that game sets both the return and the volatility.
Usually as bonus funds with their own wagering, though some offers pay spin winnings straight into the cash balance.