It doesn't limit the bandwidth to that, but limits the quality of the stream.
Doesn't Netflix limit the bandwidth based on which stream you've chosen?
No. I don't know whether the Roku bases buffer size on absolute size, or play time (probably the latter), but it will retrieve the stream data at maximum speed regardless. It's not the case that the servers are sending data at the given bit rate.
Think of it like downloading a movie to your computer outright (we'll assume it's legal for the purposes of discussion). If your connection is fast, it might take 10 minutes to download, but 120 minutes to watch.
The same thing is happening with the Roku, in chunks, over and over. It fetches part of the movie as fast as it can, until the buffer is filled. At some point, the buffer empties more than it wants, and it retrieves the next chunk of movie to top the buffer off. Each time, it's downloading the data as fast as your connection will allow.
What you really need to avoid having this behavior interfere with browsing is Quality of Service. You want the acknowledgment packets sent to web servers to have a higher priority for outbound queuing than those for bulk transfers. Many standalone routers these days have some kind of QoS available, though I don't know how effective they are, nor do I think the original poster's router is one of them. I use a Linux machine for firewall/router duties, and while QoS there is rather arcane, it works quite well once set up.