The variation appears to be due to the nature of the prepositions. Prepositions which deal with location tend to be verby in character. Owing to this verb-like nature, they can act as predicators in the clause. As further evidence, these prepositions can be preceded directly by tense and modal markers:
Yu shuda ina di hous.
2SG should in DET house
'You should have been in the house.'
Dem wehn ina di bos.
3PL ANT in DET bus
'They were in the bus.'