I'm pretty sure this is a violation of most terms of service agreements. i.e. - if the internet provider found out, you might have an issue (cancel service, fines - depends on what their agreement says). Aside from not having moral issues, you should investigate to see if it could cause problems for you.

I would also be wary of 1) slowing down the network and 2) what if they (or their guests) accidentally do something illegal (or intentionally - but that's a whole 'nother issue).

Since they're your landlords, I feel like they've put you in sort of an odd position. I know I wouldn't do it - heck, we created a separate guest network (with less access) to use with 1) less trusted devices and 2) with guests (we love 'em but....).