Here's why: when people ask first, (instead of just posting about you and either A - never telling you or B - emailing you just to let you know), 99% of the time it means that they want a free sample in order to feature it. And 99% of the time, that sounds like a scam to me.
I have nothing against these people. They are entrepreneurs just like myself, but personally I feel like it is just a waste of my time and energy that yields little to no viable results in sales, which is why I'm here to begin with. I love making jewelry and I love working for myself - but only because it puts the food on the table and the art on the walls, you know? I do not love giving away free product.
I have tried this route before in the biggest way possible - I tried the Sampler. If you don't know what it is, go visit their website to learn how they charge people for a box of samples that are sent in for free by contributors like myself. It's a great sounding idea - get a sample of your product in front of potential customers who may tell their friends how great it is and how everyone they know should visit your website to buy your product. Not so... I basically spent a bunch of money and a ton of time to come up with 75 free necklaces (all handmade for no profit) to donate to the Sampler, which supposedly has media distribution and all sorts of other perks that they talk big about, and got not one sale from it. Not one. I've heard mixed reviews about the sampler but it did not work for me and I paid a high price to learn that lesson.
There are exceptions to this, of course. I have sent free product for blog contests but only to blogs that I personally follow and hold in high regard. I have also traded product for advertising space which makes sense and I'm all for, because it's more of a trade and less of a bribe, but again it's only for blogs that I personally know and who have other advertisers that I know and respect.
All I'm saying is, if you are asked permission to be featured on someone's blog, be wary! Being written about on a blog is nice, but you shouldn't have to pay for it with free product. Visit the blog first and see if it is worth giving away your merchandise. Is your free product going to turn into dollar signs? Find out about their stats - are you going to get 50 hits? 250 hits? 1000? Don't be afraid to contact people they've "reviewed" in in the past few months to see if they felt like it was worth it (ask direct questions though - don't be too general or you may never get a response). If your gut tells you that it's not worth it, it may not be. If your gut says "heck yes!" then go for it!