Assumptions of the models for haploid and diploid selection
Unless I am missing something the equations you have posted are incorrect. The second equation should be:

$$p' = \frac{p^2 W_{AA} + p(1-p)W_{AB}}{p^2W_{AA}+ 2p(1-p)W_{AB}+(1-p)W_{BB}} $$

$p'$ is not interpreted as $\frac{dp}{dt}.$

$p'$ is the frequency of allele A in the next generation. If zygotic frequencies via random mating are 

$p^2(AA) + 2pq(Aa)+q^2(aa) = 1$

then all of AA individuals and 1/2 the Aa gametes bear the A allele. So we can find $p$ in the next generation as 

$p^2+pq = p^2+p(1-p) = p^2+p-p^2 = p$ which shows that in a Hardy-Weinberg population the frequencies are stable. 

The assumptions here are those underlying the HW model, which includes infinite population size (hence constant).