4-16V SSR control pin with 24V
Typically good SSRs use something like a constant current circuit at the input. Here is the input characteristic curve for a 4-32V Panasonic SSR AQA221VL from the datasheet: 



As you can see, the current changes little from 5V to 32V, drawing what the internal LED needs via a crude linear current regulator and little more. 

Cheaper ones (especially those with limited input voltage range) probably use a crappy Zener shunt regulator which just burns the excess current up in an internal resistor. The current will rise faster than with a resistor in this kind of circuit. Yours is probably of the cheaper type. 

Since your datasheet is a bit light on information, if you have a multimeter and a bench supply you can easily characterize the one you have and determine a suitable series resistor value that will be well within the normal operating range of the SSR. Of course the manufacturer could change the internal circuit without telling your on future orders. 

You could also just put an LM78M05 on each SSR and then it would behave more like the good SSR.