To deny meditation as a religious practice demonstrates anti-religious bias, as opposed to critical thinking. The fact nonreligious people can enjoy meditation does not take away from the fact it is a tangible accomplishment and advancement for humanity.
If meditation is brainwashing, then every human experience is brainwashing.
The prolonged ritual dancing invented by religious shamans for religious purposes is not brainwashing either, but it is mind altering.
Ritual-sexual practices, invented by different religious people in different cultures, is also mind altering, but not brainwashing.
The existence of brainwashing is extremely controversial for sociologists. In my Sociology of Religion class, we read peer-reviewed articles both for and against the existence of brainwashing. They can't figure it out because there is no hard evidence for it.