First of all because they’re not nearly as entertaining to your recipients as you think, secondly because you are putting someone’s email address on a list they cannot control, and because you know them well enough to send them one of these damn things and don’t know any better, presumably their primary address too.
In addition to adding email clutter, virus risk, wasted time, whenever you type an email address into anything other than an email you’re composing to that person (unless it’s an effing chainletter), you elevate the odds of ruining that person’s email account with spam. Why do you think these operations are free? You’re also handing over their IP address (and letting them confirm to the e-card people that their email address is indeed a “live one” if they view remote pictures in their email), and even though you may think it’s just an IP address, so what you think, it’s just plain rude to do to someone you are trying to spread holiday cheer to or whatever.
You want my idea of an acceptable e-card? I suppose emailing someone with a blank body and just a subject saying “happy whatever” and sending it is the way to go. If you have to include something in the body make it a fine girly picture, barring Freudian issues.
Oh what, because you used some high-brow-looking site like Jacquie Lawson that makes it okay? No it doesn’t – it’s not your email address to play games with. Stop it. I’m looking at you, in-laws.