Question about fans... why is it dangerous to block the suction side of axial fans, but not squirrel cage fans?

I am guessing that a squirrel cage motor allows slippage between the motor`s rotating electromagnetic force field and it`s drive shaft without drawing excessive current where as an axial fan`s motor does not. Blocking the air input to either fan would necessitate more power and thus more Amps to maintain the fan`s RPM if no slippage is allowed to occur.