I came up with a workaround, in case anyone else has this issue. Instead of the link tags, put this -

@Html.Raw("<link>")http://@item.GetFullNiceUrl()@Html.Raw("</link>")

Here's the full rss code from the razor macro...

<?xml version="1.0" encoding="UTF-8"?>
<rss version="2.0">
<channel>
    <title>@feedTitle</title>
    <link>http://@blog.GetFullNiceUrl()</link>;
    <description>@description</description>
    <pubDate>@lastUpdated.ToString("r")</pubDate>
    <lastBuildDate>@lastUpdated.ToString("r")</lastBuildDate>
    <language>en</language>
    @{
        foreach (var item in blogPosts)
        {
            <item>
                <title>@item.Name</title>
                @Html.Raw("<link>")http://@item.GetFullNiceUrl()@Html.Raw("</link>")
                <pubDate>@DateTime.Parse(item.GetProperty("PostDate").Value).ToString("r")</pubDate>
                <guid>@item.NiceUrl</guid>
                <description><![CDATA[<span>@item.GetProperty("description").Value</span>]]></description>
            </item>
        }
    }
</channel>
</rss>