Some cam sites have a voyeur section that's meant for that sort of thing (Unless that site has a rule about "No sleeping" under any circumstances), which means that members get to watch the model do everyday things around the house, without the model having to interact with you at all.
If you're referring to models actually falling asleep on cam, and they're not in a voyeur section and not doing an intentional sleep show, yeah, that looks pretty bad.
If they're doing an intentional sleep show, even though most members probably wouldn't be into that (And yeah, when members hover the preview thumbnail of the chatroom on the main homepage, it just looks bad), I don't understand how it's "wasting the member's time." If the model states in their room topic that it's a sleep show, you don't have to stick around to type "What the fuck is this bullshit?" in their chat.
You have the choice to leave the room immediately, and spend your money elsewhere. Maybe that model had cammed awake earlier that day, and now they figure that since they're about to go to sleep anyway, they'll offer a sleep fetish show to members who'd be willing to tip for a "Sleeping Beauty." Again, if they're gonna do that, I really think they need to have that placed in a 'voyeur cams' area of the site with #voyeur #sleeping hashtags or whatever, as well as a description in the room topic.
I could understand you being pissed and feeling your time was wasted if, say, it was a GOLD show you'd chipped in for (or a private show), and then once the show began the model all of a sudden goes to lay down and falls asleep. Hell, I could even understand your frustration if every time you visit this model's room, it's always during one of her sleep shows...and never when she's awake. Does she have a cam schedule posted? I don't know...are there models who do strictly sleep shows (or voyeur shows) without ever offering the live chatting interaction?