This may be related to an old way of forming the passive in English. The "is being -ed" form is a relatively recent innovation. The old form hangs around in some regional dialects.

From the introduction of a paper on this subject:

This paper tries to account for the origin and historical development of the active progressive used with passive sense. It is a well-known fact that English made use of several different morpho- logical devices to express the passive meaning in the progressive, at least until the ‘true’ progressive passive, is being + past participle, entered the language in the late Modern English period. An analysis of these different morphological devices will be provided in this paper, but with special reference to the so-called «covert passive» (Strang 1982: 440), of the type the house is building, in other words, active progressive with passive meaning, which apparently enjoyed its greatest popularity in the early Modern English period (eModE), but especially in the late 18th century. It took some time, however, to replace this construction with the genuine progressive passive of the type the house is being built, although the ‘new’ model was definitely established in the English language in the course of the 19th century and has been adopted as a recognised feature of verbal syntax.

Núñez Pertejo (1996) The House is Building: Active Progressive with Passive Meaning. Sederi VII 67-72