The fact that it is incorporated into the plant chromosomes is irrelevant, as there is experimental evidence that such DNA released from GM plants can transform soil bacteria and fungi. There is no direct experimental evidence yet that transgenic DNA from plants can gain access into bacterial cells or mammalian cells, but that is because this possibility has never been investigated. Furthermore, the T-DNA itself into which the CaMV promoter is spliced has recombinogenic left and right borders showing substantial analogy to the recombination hotspot in the CaMV promoter. That increases the likelihood that the CaMV promoter in DNA of transgenic plant material taken into cells may become integrated into cellular genomes. Admittedly, the relevant experimental investigations have not yet been done, but this possibility cannot be dismissed a priori, and should be addressed appropriately.
Transposable elements are in all genomes, as are relict viral sequences.
However, they are generally regulated by mechanisms not yet fully understood, especially if they have entered the genome a long time ago. Methylation and gene silencing is one major mechanism for preventing mobility. However, it is known that stress can induce transpositions, and it is also known that new elements entering the genome tend to jump around until they become 'tamed'. The integrated transgenic DNA may be similar to new invasive elements in the genome. As I have already stated, CaMV promoter within the intact virus will not gain access to mammalian cells because the host specificity is determined by the viral coat.