Typically, online casinos set aside a number of slot games eligible for the free spins bonus. Eligible for free spins awarded dr slot games usually count as part of the wagering requirements, if not fulfilled, you could quickly be disqualified, or any winnings thereof declared null and void.
The reason behind this is to safeguard online casinos from making huge losses as a result of excess payouts. Therefore, they tend to choose low volatile slots where the wins are relatively low.