I think the cards are lovely, but then again I'm a sucker for cute doggy and kitty pictures.
There are multiple holidays in December, I'm sure the cards go to more than just Christians, so what's the big deal?
At the paper where I used to work, I sent cards out to my clients. I sent general season's greetings or happy holidays cards - in most cases, the cards were going to more than one person at a company, and I didn't know what religion everyone was...I covered everything.
(I did send Chanukah cards to a couple customers whom I knew were Jewish.
)
I'm Jewish and I don't get offended if someone says "Merry Christmas," or "Happy Kwanzaa" or anything like that. I have several neighbors who put up Nativity scenes in their yards. As long as they don't put one up in my yard it's fine with me.
The only way I'd get offended at a Christmas card someone sent me would be if someone knew I was Jewish and sent me a super-religious Christmas card and wrote a message trying to convert me.