Good question! I'm not pretty sure but I think here the imperfect form is used because the act probably is being done at the present moment and the stress is on the act itself but not when it will be done. Moreover when the verb is in present and there is no other verb in the sentence then we prefer the imperfect verbal aspect.
хляб. (I'm buying bread.) but:
Трябва да купя
хляб. (I have to buy bread.),
Искам да купя
хляб. (I want to buy bread.)
If you use the imperfect verbal aspect in the last two examples then that requires more information (because the imperfect shows that the act is done more times or it takes longer)
Трябва да купувам
хляб (всеки ден) - I have to buy bread (every day).
(Задължително) трябва да купувам
хляб (от този магазин). - It's compulsory for me to buy bread from this shop.
Now I got a bit confused...