Green Dot says I'm violating their trademark, but I don't think so. Advice?

I have a blog post on one of my websites about how to buy stuff online if you don't have a credit card. One of the options I discuss is buying a Green Dot card and loading it with one of their moneypaks. I give step by step instructions on how to do all this, as well as tips on how to make the process go smoothly.

The article is in no way disparaging toward Green Dot, no affiliation with Green Dot is claimed or implied, I'm not using their logo or marketing slogans, and my content is 100% original. I don't think I've even made the effort to get any backlinks to this particular post, and I've certainly never used "green dot" as anchor text in a backlink. They probably discovered it because it links to their site.

I got an email a few days ago from Green Dot threatening legal action if I don't remove the content within 10 days. The gist of the email is this:

"All references to trademarked Green Dot and/or MoneyPak must be immediately removed from the website as you are not authorized to use our trademark and are in violation of law."

I think they're trying to bully me into removing the content, when in fact they don't have a legal leg to stand on. But I'd like to perform a reality check by getting a second opinion before I reply to them.

Here's why I think they're full of it:

1) If you perform a search for "green dot", there are a surprising number of companies and organizations using "Green Dot" in their name. If they're really concerned about trademark violation, seems to me that that's a more logical place to start.

2) The blog post also mentions companies like Wal-Mart, Radio Shack, Mastercard, CSV, etc. etc. Using Green Dot's logic, those would be trademark violations also (not).

3) Since my post isn't defamatory, isn't stealing their content, and doesn't claim any affiliation with Green Dot, doesn't it fall under the protection of a little thing called the Right To Free Speech? Isn't it simply what you'd call social media?

I did some online research and discovered that Green Dot has kind of a serious reputation management problem. The customers who have had bad experiences with them are really pissed off and very vocal, and it isn't difficult to find their angry posts on consumer review sites like ripoffreport.com. Green Dot has a much bigger problem than my blog post, which doesn't even show up in search results for any of their keywords.

I think their email to me was someone's really sloppy attempt at reputation management, and nothing more. What do you think?