Now lets deal with the HTML issue you bring forward in the context of theme hacking. We blogger cannot edit themes and templates underlying themes on wordpress.com blogs because we are on a multiuser blogging platform. All blogs wearing the same themes are in essence using the same underlying template. Only Staff can access those theme and template files and edit them because every edit made affects all blogs wearing the same theme.
Continuing with the HTML issue. We frequently find that those who want to insert codes they consider to be HTML are actually attempting to post affiliate links and other advertising links or e-commerce shopping cart links or other links that drive traffic to third party sites sites. As the software is programmed to strip those links out they post for help.Well, that's because we agreed to the Terms of Service here in order to get a free hosted wordpress.com blog.
No blogger initiated advertising, retailing or reselling the work created or services provided by anyone other than yourself is allowed. E-commerce transactions via shopping carts and the like cannot be conducted on wordpress.COM blogs.
The only advertising exceptions are for high traffic blogs that qualify for and are accepted into the Ad Control program, and for extremely high traffic blogs that qualify and are accepted into the paid VIP hosting program.
The only exceptions with regard to affiliate links are found here:
Affiliate marketing blogs: Blogs with the primary purpose of driving traffic to affiliate programs and get-rich-quick schemes (“Make six figures from home!!”, “20 easy steps to top profits!!”, etc). This includes multi-level marketing (MLM) blogs and pyramid schemes. To be clear, people writing their own original book, movie or game reviews and linking them to Amazon, or people linking to their own products on Etsy do NOT fall into this category. http://en.wordpress.com/types-of-blogs/
I hope the time I invested into typing all of this benefits you and all wordpress.com members who read this thread.
There are two wordpress options - free hosted (wordpress.com) and self hosted (wordpress.org). if you want to have complete control over a blog hire a web host, get a free software install and do as you wish.