The web/app reader only provides access to previews and pre-publication novels. Paying members get all prepub novel parts but once a novel has been published for a week or two it disappears from the prepub system and you have to buy it as an epub etc for continued digital access. Premium subscribers get a monthly free credit for premium epubs and a discount on buying more credits.
There are usually numerous novels free at any one time but they are not all first volumes. The money people pay helps fund some high quality translations and supports the light novel industry so I think it is good value but I understand the frustration of many of the free volumes being for later volumes only. If you want to discuss how to improve the experience for new subscribers see the thread at https://forums.j-novel.club/topic/506/don-t-remove-vol-1-of-any-novel-series/