The fixed line slot game have the permanently enabled paylines (or ways) to win. I think, the more paylines on slot machine are enabled, the more chances to win.
I think, that gambler softwares do correctly when fix all lines in slot game. I do not see sense to play only on one payline in the slots where 50 lines are available. These are the multi-line slots, and it means that you can play all paylines per spin. It's good chance to increases your wins. But there are players who like "to try to play" multi-line slot game on one or two paylines. Then for them it is important to know, that there are games which exclude an opportunity of a choice of paylines to play. Below see some online slot games with the fixed paylines.