Should window well for basement window be attached to the house?
According to 2012 International Residential Code (IRC), there should be adequate drainage in the bottom of the well. So if it's installed to meet this code, then you shouldn't have any problems with the well flooding. The 2009 version of IRC didn't seem to have this bit of code (at least not that I found), so if your area hasn't adopted 2012 IRC you might have a problem.

International Residential Code 2012
Chapter 3 Building and Planning
Section 310 Emergency Escape and Rescue Openings
R310.2 Window wells.
R310.2.2 Drainage. Window wells shall be designed for proper drainage by connecting to the building’s foundation drainage system required by Section R405.1 or by an approved alternative method.

If they didn't install proper drainage, I'm not sure how it passed inspection (other than not being inspected). I might contact the company that installed the wells, and try to get them to install proper drainage, or figure out if they installed an alternative drainage method.
Aside from that, you could install drainage yourself, or keep a submersible pump on hand and at the ready.