Because Heaven wasn't actually Heaven (e.g. in the Christian sense of an other-worldly place filled with spirits and souls), it was actually a Time Lord storage device filled with the mind-states of the recently dead, recorded and captured using technology...
by the Master/Missy as part of (what else?) an elaborate plot against the Doctor.
...
SEB: You're dead and, and this is what's next.
DOCTOR: A Gallifreyan hard drive. Time Lord technology.
That being the case, there seems to be no special restriction on a robotic intelligence being uploaded into the mind-sphere, any more than an organic intelligence can be uploaded.
