Because they used the word Stack in their name? StackLead and StackOverflow seem sufficiently different to me to not be confusing. The use of the word stack is fairly common in other companies and projects like saltstack, openstack, stackmedia, etc.
StackOverflow also provides job-finding/head-hunting services, and yes, they could probably successfully argue that they have ownership of "the Stack name" with regards to the space of job placement, given they've exercised that name into many variants of Stack*.
StackLead isn't providing job-finding/head-huntiong services though, from my reading of their site they are more suited to sales lead management.
A quick search of USPTO shows plenty of other companies using stack in their registered trademarks. The search results don't appear to be linkable but starting here -> http://tmsearch.uspto.gov/ it shouldn't be too hard to find plenty.
